CHAPTER IX
DICHLOROETHYLSULFIDE
“MUSTARD GAS”
The early idea of gas warfare was that a material, to be of value as a war gas, should have a relatively high vapor pressure. This would, of course, provide a concentration sufficiently high to cause casualties through inhalation of the gas-ladened air. The introduction of “mustard gas” (dichloroethylsulfide) was probably the greatest single development of gas warfare, in that it marked a departure from this early idea, for mustard gas is a liquid boiling at about 220° C., and having a very low vapor pressure. But mustard gas has, in addition, a characteristic property which, combined with its high persistency, makes it the most valuable war gas known at the present time. This peculiar property is its blistering effect upon the skin. Very low concentrations of vapor are capable of “burning” the skin and of producing casualties which require from three weeks to three months for recovery. The combination of these properties removed the necessity for a surprise attack, or the building up of a high concentration in the first few bursts of fire. A few shell, fired over a given area, were sufficient to produce casualties hours and even days afterwards.
Mustard gas, chemically, is dichloroethylsulfide (ClCH₂CH₂)₂S. The name originated with the British Tommy because the crude material first used by the Germans was suggestive of mustard or garlic. Various other names were given the compound, such as “Yellow Cross,” from the shell markings of the Germans; “Yperite,” a name used by the French, because the compound was first used at Ypres; and “blistering gas,” because of its peculiar effect upon the skin.
HISTORICAL
It seems probable that an impure form of mustard gas was obtained by Richie (1854) by the action of chlorine upon ethyl sulfide. The substance was first described by Guthrie (1860), who recognized its peculiar and powerful physiological effects. It is interesting in this connection to note that Guthrie studied the effect of ethylene upon the sulfur chlorides, since this reaction was the basis of the method finally adopted by the Allies.
The first careful investigation of mustard gas, which was then only known as dichloroethylsulfide, was carried out by Victor Meyer (1886). Meyer used the reaction between ethylene chlorhydrin and sodium sulfide, with the subsequent treatment with hydrochloric acid. All the German mustard gas used during 1917 and 1918 was apparently made by the use of these reactions, and all the early experimental work of the Allies was in this direction.
Mustard gas was first used as an offensive agent by the Germans on July 12-13, 1917, at Ypres. According to an English report, the physiological properties of mustard gas had been tested by them during the summer of 1916. The Anti-Gas Department put forward the suggestion that it should be used for chemical warfare, but at that time its adoption was not approved. This fact enabled the English to quickly and correctly identify the contents of the first Yellow Cross dud received. It is not true, as reported by the Germans, that the material was first diagnosed as diethylsulfide.
The tactical value of mustard gas was immediately recognized by the Germans and they used tremendous quantities of it. During ten days of the Fall of 1917, it is calculated that over 1,000,000 shell were fired, containing about 2,500 tons of mustard gas. Zanetti states that the British gas casualties during the month following the introduction of mustard gas were almost as numerous as all gas casualties incurred during the previous years of the war. Pope says that the effects of mustard gas as a military weapon were indeed so devastating that by the early autumn of 1917 the technical advisers of the British, French, and American Governments were occupied upon large scale installations for the manufacture of this material.
PREPARATION AND MANUFACTURE
The analysis of the first German shell indicated that the mustard gas contained therein had been prepared by the method published by Victor Meyer (1886) and later used by Clark (1912) in England. It was natural, therefore, that attention should be turned to the large scale operation of this method.
The following operations are involved: Ethylene is prepared by the dehydration of ethyl alcohol. The interaction of hypochlorous acid (HClO) and ethylene yields ethylene chlorhydrin, ClCH₂CH₂OH. When this is treated with sodium sulfide, dihydroxyethyl sulfide forms, which, heated with hydrochloric acid, yields dichloroethyl sulfide. Chemically, the reactions may be written as follows:
CH₃CH₂OH = CH₂ : CH₂ + H₂O CH₂ : CH₂ + HClO = HOCH₂CH₂Cl 2HOCH₂CH₂Cl + Na₂S = (HOCH₂CH₂)₂S + 2NaCl (HOCH₂CH₂)₂S + 2HCl = (ClCH₂CH₂)₂S + 2H₂O
Without going into the chemistry of this reaction, which is thoroughly discussed by Gomberg[18] (see also German Manufacture), it may be said that this “procedure proved to be unsuitable for large scale production” (Dorsey). As Pope remarks, “That he (the German) should have been able to produce three hundred tons of mustard gas per month by the large scale installation of the purely academic method (of Meyer) constitutes indeed ‘a significant tribute to the potentialities represented by the large German fine chemical factories.’” It is true that a great deal of experimental work was carried out by the Allies on this method, but further study was dropped as soon as the Pope method was discovered.
[Footnote 18: J. Am. Chem. Soc. =41=, 1414 (1919).]
The first step in advance in the manufacture of mustard gas was the discovery that ethylene would react with sulfur dichloride. While American chemists were not very successful in their application of this reaction, either in the laboratory or the plant, it was apparently, according to Zanetti, the only method used by the French (the only one of the Allies that manufactured and fired mustard gas). The plant was that of the Société Chimique des Usines du Rhone and was started early in March, 1918, with a production of two to three tons a day. In July it was producing close to twenty tons a day. The plant was being duplicated at the time of the Armistice, so that probably in December, 1918, the production of mustard gas by the dichloride process would have reached about 40 tons. Zanetti points out, however, that the process involved complicated and costly apparatus and required considerable quantities of carbon tetrachloride as a solvent. It is for this reason that the Levinstein process would have been a tremendous gain, had the war continued.
About the end of January, 1918, Pope and Gibson, in a study of the reaction originally used by Guthrie, found that the action of ethylene upon sulfur chloride (S₂Cl₂) at 60° yielded mustard gas and sulfur:
2CH₂ : CH₂ + S₂Cl₂ = (CH₂ClCH₂)₂S + S
The reaction at this temperature caused the separation of sulfur; this occurred after the product stood for some time or immediately if it was treated with moist ammonia gas. While this process was put into commercial operation, both in England and America, it offered considerable difficulty from an operating standpoint. The sulfur would often separate out and block the inlet tubes (ethylene). While it is comparatively easy to remove the mustard gas from the separated sulfur by decantation, a certain amount always remains with the sulfur. It is almost impossible to economically remove this, and its presence adds to the difficulty of removing the sulfur from the reactors; the men engaged in this operation almost always become casualties.
[Illustration: FIG. 29.—The Levinstein Reactor as Installed at Edgewood Arsenal.]
It was especially important, therefore, when Green discovered that, if the reaction was carried out at 30°, the sulfur did not settle out but remained in “pseudo solution” in the mustard gas (Pope) or as a loose chemical combination of the monosulfide (mustard gas) with an atom of sulfur (Green). This material has all the physiological activity of the pure monosulfide, while the enormous technical difficulties of handling separated sulfur are entirely obviated by this method of manufacture. To carry out the reaction Levinstein, Ltd., devised the Levinstein “reactor.” The apparatus is shown in Fig. 29. The process consists essentially in bringing together sulfur chloride and very pure ethylene gas in the presence of crude mustard gas as a solvent at a temperature ranging between 30-35° C. A supply of unchanged monochloride is constantly maintained in the reacting liquid until a sufficiently large batch is built up. Then the sulfur monochloride feed is discontinued and the ethylene feed continued until further absorption ceases. By controlling the ratio of mustard gas to uncombined monochloride, the reaction velocity is so increased that the lower temperature may be used.
The product thus obtained is a pale yellow liquid which deposits no sulfur and requires no further treatment. It is ready for the shell filling plant at once. The obvious advantage of this method led to its adoption in all American plants started for the manufacture of mustard gas (Edgewood, Cleveland and Buffalo).
ETHYLENE
It was known from the work of certain French chemists that in the presence of such a catalyst as kaolin, ethyl alcohol is dehydrated at an elevated temperature to ethylene. The process as finally developed by American chemists consisted essentially in introducing mixtures of alcohol vapor and steam, in the ratio of one to one by weight, into an 8-inch iron tube with a 3-inch core, in contact with clay at 500-600° C. The use of steam rendered the temperature control more uniform and thus each unit had a greater capacity of a higher grade product. The gaseous products were removed through a water-cooled surface condenser. One unit of this type had a demonstrated capacity of 400 cubic feet per hour of ethylene, between 92 and 95 per cent pure, while the conversion efficiency (alcohol to ethylene) was about 85 per cent. The Edgewood plant consisted of 40 such units. This would have yielded sufficient ethylene to make 40 tons of mustard gas per 24-hour day.
The English procedure consisted in the use of phosphoric acid, absorbed onto coke. An American furnace was designed and built which gave 2,000 cubic feet per hour of ethylene, with a purity of 98 to 99 per cent. This furnace was not used on a large scale, because of the satisfactory nature of the kaolin furnaces.
[Illustration: FIG. 30.—Experimental Installation for the Production of Ethylene by Kaolin Procedure. Capacity 400-600 cu. ft. Ethylene per hr.]
SULFUR CHLORIDE
Since chlorine was prepared at Edgewood, it was logical that some of this chlorine should be utilized in the preparation of sulfur chloride. The plant constructed consisted of 30 tanks (78 inches in diameter and 35 feet long), each capable of producing 20,000 pounds of monochloride per day. The tanks are partially filled with sulfur and chlorine passed in. The reaction proceeds rapidly with sufficient heat to keep the sulfur in a molten condition. If the chlorine is passed in too rapidly, the heat generated may be sufficient to boil off the sulfur chloride formed. Hence water pipes are provided so that a supply of cold water may be sprayed upon the tanks, keeping the temperature within the proper limits.
[Illustration: FIG. 31.—Row of Furnaces for the Preparation of Ethylene.]
In the manufacture of one ton of mustard gas, about one ton of sulfur chloride and a little less than half a ton of ethylene (12,640 cubic feet) are required.
GERMAN METHOD OF MANUFACTURE[19]
[Illustration: FIG. 32.—Preparation of Ethylene at Badische Anilin und Soda Fabrik. 60 units.]
[Footnote 19: Norris, _J. Ind. Eng. Chem._, =11=, 821 (Sept., 1919).]
“PREPARATION OF ETHYLENE—The gas was prepared by passing alcohol vapor over aluminum oxide at a temperature of 380° to 400°. The details of the construction of one of the furnaces are given in Figs. 32 and 33. The furnaces were very small and sixty units were needed to furnish the amount of gas required. The tubes containing the catalyzer were made of copper and were heated in a bath of molten potassium nitrate. It was stated that the catalyzer was made according to the directions of Ipatieff, and that its life was from 10 to 20 days. The gas produced was washed in the usual form of scrubber. The yield of ethylene was stated to be about 90 per cent of the theoretical.
[Illustration: FIG. 33.—Ethylene Production at Badische Anilin und Soda Fabrik. 1 unit.]
[Illustration: FIG. 34.—Chlorhydrin reaction kettle at Badische Anilin und Soda Fabrik. 16 units.]
“PREPARATION OF ETHYLENE CHLORHYDRIN—The reaction was carried out in a cylindrical tank resting on its side. The tank was furnished with a stirrer and was insulated by means of cork in order to prevent the transfer of heat from the atmosphere to the inside. Enough chloride of lime was introduced into the tank to furnish 500 kg. of available chlorine, together with 5 cu. m. of water. At first, about 20 cu. m. of carbon dioxide were led into the mixture, next ethylene, and later carbon dioxide and ethylene simultaneously. The rate of absorption of ethylene was noted and when it slackened, more carbon dioxide was added. Fuller details as to the addition of the two gases were not given as it was stated that it was a matter of judgment on the part of the workman who was carrying out the operation. The reaction should be carried out at as low a temperature as possible, but it was found impossible to work below 5° with the apparatus employed in this factory. The temperature during the reaction varied between 5° and 10°. In order to maintain this temperature, the solution was constantly pumped from the apparatus through a coil which was cooled by brine. When ethylene was no longer absorbed and there was an excess of carbon dioxide present, the solution was tested for hypochlorous acid. The time required for the introduction of ethylene was between 2 and 3 hrs.
“The contents of the apparatus were passed through a filter press by means of which the calcium carbonate was removed. The solution thus obtained contained from 10 to 12 per cent of ethylene chlorhydrin. It was next distilled with steam and a distillate collected which contained between 18 and 20 per cent of chlorhydrin. The yield of chlorhydrin was from 60 to 80 per cent of that calculated from the ethylene used.
[Illustration: FIG. 35.—Mustard Gas Manufacture at Leverkusen. Layout for Chlorination of Thiodiglycol.]
“PREPARATION OF DIHYDROXYETHYLSULFIDE—To prepare the hydroxysulfide, the theoretical quantity of sodium sulfide, either in the form of the anhydrous salt or as crystals, was added to the 18 to 20 per cent solution of chlorhydrin. After the addition of the sulfide, the mixture was heated to about 90° to 100°. It was then pumped to an evaporator, and heated until all the water was driven off. The glycol was next filtered from the salt which separated, and distilled in a vacuum. The yield of glycol was about 90 per cent of the theoretical, calculated from the chlorhydrin.
“PREPARATION OF DICHLORETHYLSULFIDE—The thiodiglycol was taken from the rail to two large storage tanks and thence drawn by vacuum direct to the reaction vessel. Each reaction vessel was placed in a separate cubicle ventilated both from above and below and fitted with glass windows for inspection. The vessels themselves were made of 1¼ in. cast iron and lined with 10 mm. lead. They were 2.5 m. high and 2.8 m. in diameter. These tanks were jacketed so that they could be heated by water and steam, and the reaction was carried out at 50°. The hydrochloric acid coming from the main pipe was passed through sulfuric acid so that the rate could be observed, and passed in by means of 12 glass tubes of about 2 cm. diameter. The rate of flow was maintained at as high a rate as possible to procure absorption. The vapors from the reaction were led from the vessel through a pipe into a collecting room, and then through a scrubber containing charcoal and water, through a separator, and then, finally, into the chimney. These exhaust gases were drawn off by means of a fan which was also connected with the lower part of the chamber in which the reaction vessels were set, so that all the gases had to pass through the scrubber before going to the chimney. When the reaction was completed, the oil was removed by means of a vacuum, induced by a water pump, into a cast iron washing vessel.
“The hydrochloric acid layer was removed to a stoneware receiver, also by vacuum. A glass enabled the operator to avoid drawing oil over with the acid. The pan was fitted with a thermometer to the interior as well as to the jacket. For testing the material during reaction, provision was made for drawing some up by vacuum to a hydrometer contained in a glass funnel. The final test at this point read 126° Tw. Another portion could be drawn up to a test glass and hydrochloric acid passed through it in full view. A float contained in a glass outer tube served to show the level of the liquid in the vessel. The pans in which the operation is carried on, as well as those employed for washing and distilling the product, were of a standard pattern employed in many other operations in the works.
“The washer consisted of a cast iron vessel, lead lined, and was 2.5 m. in diameter, 2 m. deep, and fitted with a dome cover and stirring gear. Lead pipes served for the introduction of sodium carbonate solution and water. Similar pipes were fitted for drawing these off by means of a vacuum. A manhole on the cover, with a flat top, was fitted with light and sight glasses to which were fitted a small steam coil for keeping them clear. The washed oil is drawn off to a distillation still, which is a cast iron vessel homogeneously lead coated, 1.5 m. in diameter and 2 m. deep, fitted with a lead heating coil and connected through a spiral lead condenser and receiver to a vacuum pump. The water is distilled from the oil at a pressure of from 62 to 70 mm. absolute pressure. When dried, the oil is sent by vacuum to a mixing vessel, similar in most respects to the washing vessel, in which it is mixed with an appointed quantity of solvent, which, in this factory, was usually chlorobenzene but occasionally carbon tetrachloride. The relative quantities varied with the time of year, and instructions were sent from Berlin on this point. Thence the mixture was passed to a storage tank and into tank-wagons.”
AMERICAN METHOD OF MANUFACTURE
The Chemical Warfare Service investigated carefully the three methods (German, French, and English) and finally adopted the Levinstein process. The following discussion is taken from a report originally made during construction, Sept., 1918.
The Levinstein reactor consisted of a jacketed and lead-lined vessel or steel tank, 8 feet 5 inches in diameter and 14 feet tall. The reactor contained 1,400 feet of lead pipe (outside diameter 2⅜ inches), made up into five coils, giving a total cooling surface of 1,200 square feet. The finished charge of such a reactor is 12 tons.
Ethylene was introduced through lead injectors, of which there were 16, each suspended from its own opening in the top and hanging so that the end of the injector tube was 12 inches from the bottom of the reactor. The nozzle of the injector was ³/₁₆ inch outside diameter and ethylene was introduced through it at 40 pounds pressure.
In starting the reaction, enough sulfur chloride was introduced into the reactor to cover the central nozzles. Ethylene was now introduced, and as the reaction proceeded sulfur chloride was added in sufficient quantities to give a high rate of reaction. Brine or cold water was introduced through the cooling coils and jacket to keep the reacting temperature at 35° C.
When the charge was completed, the ethylene was turned off so that only a small amount bubbled through the nozzles and the charge syphoned off to the settling tank. These were constructed of iron, 8 feet in diameter and 19 feet tall. They were provided with iron coils by which the liquid may be cooled down, or the sulfur, which precipitates in the bottom, melted. The tank was large enough to hold six complete charges of mustard gas and all the sulfur from these charges was allowed to accumulate before removal of the sulfur. The supernatant mustard gas was drawn off from above this sulfur to storage tanks.
Among the factors which influence the reaction are the following:
A temperature of over 60° C. in lead will decompose the product slowly when sulfur chloride is present.
The presence of iron decomposes the product rapidly at a temperature of 50° C. and probably at a considerably lower temperature.
The purity of the product is dependent upon the time of reaction. There is always a slow reaction between the mustard gas and sulfur chloride, and because of this the charge should be completed in 8 hours.
In general the more sulfur that comes out of the solution, the better is the product. Temperature has a marked effect on the separation of sulfur. In order to entirely remove the sulfur from the product it was the custom to increase the temperature at the close of the reaction from 55° to 70° C. This, however, caused plugging of the lines and the reactor.
PROPERTIES
Dichloroethylsulfide (mustard gas) is a colorless, oily liquid, which has a faint mustard odor. The pure material is said to have an odor very suggestive of that of water cress. While the odor is more or less characteristic, it is possible to have extremely dangerous amounts of the gas in a neighborhood without being detected through its odors. It still seems to be an open question whether mustard gas paralyzes the sense of smell. One can find opinions on both sides.
Mustard gas boils at 215°-217° C. at atmospheric pressure, so that it is at once seen to be a very persistent gas. It distills without decomposition at this temperature but is best purified by vacuum distillation, or by distillation with steam. A still for the vacuum distillation of mustard gas has been described by Streeter.[20]
[Footnote 20: _J. Ind. Eng. Chem._, 11, 292 (1919).]
Mustard gas melts, when pure, at 13° to 14° C. (The ordinary summer temperature is 20°-25° C.). The ordinary product, as obtained from the “reactor,” melts from 9°-10° C. In order that the product in the shell might be liquid at all temperatures, winter as well as summer, the Germans added from 10 to 30 per cent of chlorobenzene, later using a mixture of chlorobenzene and nitrobenzene and still later pure nitrobenzene. Carbon tetrachloride has also been used as a means of lowering the melting point. Many other mixtures, such as chloropicrin, hydrocyanic acid, bromoacetone, etc., were tested, but were not used. The effect on the melting point of mustard gas is shown in the following table:
MELTING POINT OF MUSTARD GAS MIXTURES
---------+--------------+---------------+--------------- Per Cent | Chloropicrin | Chlorobenzene | Carbon Added | | | Tetrachloride ---------+--------------+---------------+--------------- 0 | 13.4° C. | 13.4° C. | 13.4° C. 10 | 9.8 | 8.4 | 9.8 20 | 6.3 | 6.4 | 6.6 30 | 2.6 | -1.0 | 3.1 ---------+--------------+---------------+---------------
The mustard gas as finally made by the United States contained about 17 to 18 per cent sulfur in solution. The gas was then put in shell and fired without the addition of any solvent. In actual practice this impure product seemed even more powerful in causing casualties than equal quantities of the pure mustard gas. Accordingly no redistilling as originally contemplated was actually carried out.
The specific gravity of mustard gas at 20° is 1.2741. The solid material has a slightly higher value, being 1.338 at 13°. Its vapor pressure at room temperature is very low; at 20° this value has been found to be about 0.06 mm. of mercury.
Mustard gas is practically insoluble in water, less than 0.1 per cent forming a saturated solution. The reports that a 1 per cent solution could be obtained did not consider the question of hydrolysis. Mustard gas is freely soluble in all the ordinary organic solvents, such as ligroin, alcohol, ether, chloroform, acetic acid, chlorobenzene, etc. In case the solvent is miscible with water, dilution throws out the product as an oil.
CHEMICAL PROPERTIES
Mustard gas is very slowly decomposed by water, owing to its very slight solubility. The products are dihydroxyethylsulfide and hydrochloric acid:
(ClCH₂CH₂)₂S + 2H₂O = (HOCH₂CH₂)₂S + 2HCl
Certain sulfonated oils accelerate the rate of hydrolysis, both by increasing the rate of solution and the solubility of the mustard gas. Alkalies also increase the rate of hydrolysis. Oxidizing agents destroy mustard gas. This reaction was made use of practically in that solid bleaching powder was early introduced as a means of destroying mustard gas in the field. (Fig. 9.)
Chlorinating agents (chlorine, sulfur dichloride, etc.) rapidly transform mustard gas into an inactive (non-blistering) substance. Sulfur dichloride was a valuable reagent in both laboratory and works in “cleaning up” mustard gas. This reaction also explains why the early attempts to prepare mustard gas by the interaction of ethylene and sulfur dichloride were unsuccessful. Mustard gas is probably formed, but is almost immediately chlorinated by the excess of sulfur dichloride. Sulfur chloride on the other hand has no effect on mustard gas. Chloramine-T and Dichloramine-T (the valuable therapeutic agents introduced by Dakin and Carrel for treatment of wounds) also react with mustard gas. For this reason they were advocated as treatment for mustard gas burns. But as we will see later, they were not altogether successful.
DETECTION
At first the only method of detecting mustard gas was through the sense of smell. It was then believed that concentrations which could not be detected in this way were harmless. Later this proved not to be the case, and more delicate methods had to be devised. In the laboratory and in the field these tests were not very satisfactory, because most of them depended upon the presence of chlorine, and the majority of the war gases contained chlorine or one of the other halogens. The Lantern Test depended upon the accumulation of the halogen upon a copper gauze and the subsequent heating of the gauze in a Bunsen flame. This test could be made to detect one part of mustard gas in ten million parts of air. Another field detector devised by the Chemical Warfare Service consisted in the use of selenious acid. Here again the lack of specificity is apparent, for while certain halogen compounds did not give the test, arsine and organic arsenicals gave a positive reaction and often in a shorter time than mustard gas.
[Illustration: FIG. 36.—Field Detector for Mustard Gas.]
The Germans are said to have had plates covered with a yellow composition which had the property of turning black in the presence of mustard gas. These plates were lowered into the bottom of recently captured trenches and if, after a few minutes, they turned black, the presence of mustard gas was suspected. It is also stated that the characteristic yellow paint on the olive of the mustard gas shell had the same composition, and was useful in detecting leaky shell. According to a deserter’s statement, however, reliance upon this test resulted in casualties in several instances.
A white paint has also been reported which turned red in the presence of mustard gas. This color change was not characteristic, for tests made by our Army showed that other oils (aniline, turpentine, linseed) were found to produce the same effect.
The Chemical Warfare Service was able to develop an enamel and an oil paint which were very sensitive detectors of mustard gas. Both of these were yellow and became dark red in contact with mustard gas. The change was practically instantaneous. The enamel consisted of chrome yellow as pigment mixed with oil scarlet and another dye, and a lacquer vehicle, which is essentially a solution of nitrocellulose in amyl acetate. One gallon of this enamel will cover 946,500 sq. cm., or a surface equivalent to a band 3 cm. wide on 12,500 seven cm. shell.
The paint was composed of a mixture of 50 per cent raw linseed oil and 50 per cent Japan drier, with the above dye mixture added to the required consistency. In contact with liquid mustard gas, this changes to a deep crimson in 4 seconds. Furthermore, in contact with arsenicals, this paint changes to a color varying from deep purple to dark green, the color change being almost instantaneous and very sensitive, even to the vapors of these compounds. Other substances have no effect upon the paint.
For field work, however, nothing was found equal to the trained nose, and it is questionable if any of the mechanical means described will be used in the field.
PHYSIOLOGICAL ACTION
One of the most interesting phases of mustard gas is its peculiar physiological action. This has been studied extensively, both as relates to the toxicity and to the skin or blistering effect.
TOXICITY
When one considers the high boiling point of mustard gas, and its consequent low vapor pressure, he is likely to conclude that such a substance would be of comparatively little value as a toxic or poison gas. While it is true that an important part of the military value of mustard gas has been because of its vesicant properties, the fact still remains that it is one of our most toxic war gases. The following comparison with a few of the other gases indicates this:
-----------------+---------------------- | Mg. per Liter +---------------+------ | Mice | Dogs -----------------+---------------+------ Mustard gas | 0.2 | 0.05 Phosgene | 0.3 | ··· Hydrocyanic acid | 0.2 | 0.1 Chloropicrin | 1.5 | 0.8 Chlorine | ··· | 3.0 -----------------+---------------+------
When an animal is exposed to the vapors of mustard gas in high concentration, it subsequently shows a complexity of symptoms, which may be divided into two classes:
(1) The local effects on the eyes, skin and respiratory tract. These are well recognized and consist mainly of conjunctivitis and superficial necrosis of the cornea; hyperemia, œdema and later, necrosis of the skin, leading to a skin lesion of great chronicity; and congestion and necrosis of the epithelial lining of the trachea and bronchi.
(2) The systemic effects due to the absorption of the substance into the blood stream, and its distribution to the various tissues of the body.
The most striking observation about the symptoms of mustard gas poisoning is the latent period which elapses after exposure before any serious objective or subjective effects are noted. The developments of the effects are then quite slow, unless very high superlethal doses have been inhaled.
At first it was a very serious question whether or not the temporary blindness resulting from mustard gas would not be permanent. Later, as the depth and seriousness of some of the body burns became well known, it was a seven-day wonder that no permanent blindness occurred.
The reason seems to be largely a mechanical one. The constant winking of the eyelids apparently washes the mustard gas off the eyeball and carries it away so that not enough remains to burn to the depth necessary to cause permanent blindness.
Due to the very slight concentrations ordinarily encountered in the field, resulting from a very slow rate of evaporation, the death rate is very low, probably under 1 per cent among the Americans gassed with mustard during the war.
If, on the other hand, the gas be widely and very finely dispersed by a heavy charge of explosive in the shell, the gas is very deadly. In such cases the injured breathe in minute particles of the liquid and thus get hundreds of times the amount of gas that would be inhaled as vapor. This so-called “high explosive mustard gas shell” was a German development in the very last months of the war. Its effects were great enough to make it certain that in the future large numbers of these shell will be used.
The similarity of the symptoms and pathological effects after the inhalation of large amounts of the vapor and those following an injection of an olive oil or water solution of mustard gas led Marshall and his associates to conclude that in high concentrations mustard gas is absorbed through the lungs. A further bit of evidence consists in the isolation of the hydrolysis product, dihydroxyethylsulfide, in the urine of animals poisoned by inhalation of mustard gas. This product is not toxic and is not responsible for the effects of mustard gas. Hydrochloric acid, however, does produce very definite effects upon the animal and may cause death.
From these facts Marshall[21] has proposed the following mechanism of the action of mustard gas:
[Footnote 21: Marshall, Lynch and Smith, _J. Pharmacal_, =12=, 291-301 (1918).]
“Dichlorethylsulphide is very slightly soluble in water and very freely soluble in organic solvents, or has a high lipoid solubility or partition coefficient. It would, therefore, be expected to penetrate cells very readily. Its rapid powers of penetration are practically proven by its effects upon the skin. Having penetrated within the living cell, it would undoubtedly hydrolyze. The liberation of free hydrochloric acid _within the cell_ would produce serious effects and might account for the actions of dichlorethylsulphide. To summarize, then, the mechanism of the action of dichlorethylsulphide appears to be as follows:
“1. Rapid penetration of the substance into the cell by virtue of its high lipoid solubility.
2. Hydrolysis by the water within the cell, to form hydrochloric acid and dihydroxyethylsulphide.
3. The destructive effect of hydrochloric acid upon some part or mechanism of the cell.
“Although hydrochloric acid does not penetrate cells readily and is easily neutralized by the buffer action of the fluids of the body, we might expect by flooding the body with large quantities of acid to produce some of the characteristic effects of mustard gas. Stimulation of the respiratory center is a well known effect of acid. Convulsions and salivation may be produced by injection of hydrochloric acid and we have been able to produce slowing of the heart by rapid injection of this acid.
“The delayed action of mustard gas might be explained by the formation of some compound with some constituent of the blood. However, blood taken from dogs which had been poisoned with mustard gas and were exhibiting typical symptoms at the time, injected into normal dogs produced no effect. Serum treated in vitro with mustard gas and allowed to stand and then injected into a dog, produced no effect. The fluid which is formed in the vesicle and blebs produced by the application of mustard gas to the skin produces no mustard gas effects.”
In studying the toxicity of mustard gas for dogs, it was observed that a concentration of 0.01 mg. per liter could be tolerated indefinitely. If this value is considered as a threshold value, and subtracted from the toxicity values for varying periods of time, it is found that there is a definite relation between the toxic concentration and the time of exposure. This is expressed by the formula
(C - 0.01)_t_ = K where C is the concentration observed for a given time _t_. K has the approximate value of 1.7, where _t_ varies between 7.5 and 480 minutes.
VESICANT ACTION
In addition to its toxicity mustard gas is highly important because of its peculiar irritating effect upon the skin. Its value is seen when we realize that one part in 14,000,000 is capable of causing conjunctivitis of the eye and that one part in 3,000,000 and possibly one part in 5,000,000 will cause a skin burn in a sensitive person on prolonged exposure. According to Warthin, the lesions produced by mustard gas are those of a chemical, not unlike hydrochloric acid, but of much greater intensity. The pathology of these lesions has been carefully studied and fully described by Warthin and Weller in their book on The Pathology of Mustard Gas. Our observations will therefore be confined to certain striking features of the vesicant action of this substance.
VARIATION IN SUSCEPTIBILITY OF THE SKIN
Every worker who has worked with mustard gas has noticed that some individuals are much more susceptible to skin burns from this substance than are others. Marshall made a study of 1282 men at Edgewood Arsenal, using a 1 per cent and a 0.01 per cent solution of mustard gas in paraffin oil. A small drop of these solutions was applied to the skin of the forearm of the subject and the arm allowed to remain uncovered for about 10 minutes. The presence or absence of a positive reaction is indicated by the appearance or absence of erythema 24 hours later. The results were as follows:
1% 0.01% % of Total Positive Positive 3.3 Positive Negative 55.3 Negative Negative 41.4
The test made on 84 negroes gave the following results:
1% 0.01% % of Total Positive Positive 0.0 Positive Negative 15.0 Negative Negative 78.0 Questionable Negative 7.0
“It is seen from the above tables that negroes as a race, have a much more resistant skin than white men. No negro of the 84 examined reacted to the 0.1 per cent solution, and of course none would react to a more dilute one. About 10 per cent of white men react to the 0.1 per cent solution, while 2 to 3 per cent react to the 0.01 per cent solution or are hypersensitive. About 78 per cent of the negroes fail to react to the 1 per cent solution, while only 20 to 40 per cent of the white race do not show a reaction.”
[Illustration: FIG. 37.]
The same individual may also show variations in susceptibility and this has also been studied by Marshall.
“The effect of exercise and sweating was investigated. A number of individuals were given vapor burns (one to five minutes exposure) and then exercised until in a profuse sweat, and then the same exposure to vapors made. In all cases the burn produced after exercising was more severe. Sweating produced by having the subjects place their feet in hot water, produced the same increase in susceptibility. That the moisture on the skin produced by sweating is at least partly, if not entirely, responsible for the increased susceptibility, was shown in the following way: An area of the forearm was kept moist for a few minutes with wet cotton. The sponge was then removed and two vapor tests made, one over the moist area and one over normal, dry skin. In all cases the moist burn was the more severe, in one, producing a blister where the control did not.
“The skin of different areas of the body is undoubtedly somewhat different in its susceptibility. All our tests have been applied to the forearm. The hands are considerably more resistant than the forearm. Tests made by the oil method on the forearm, chest, and back, however, indicate very little difference in susceptibility of these areas. The skin in the neighborhood of old burns has been shown to be more susceptible.
“In general, the same individual does not become more susceptible to skin burns from continued exposure to the vapor. The great number of tests which have been made on the same individual at different times and under the same conditions, indicate a remarkable constancy in reaction. A series of men who were tested at various times during a period of four months, revealed slight changes from time to time in some of the men. No man who originally reacted to only the 1 per cent solution ever reacted to the 0.01, and likewise, no man who originally reacted to the 0.01 ever failed to react to the 0.1 per cent.
“_Susceptibility of skin of animals._ The paraffin oil test was used on a number of animals and indicated that differences in susceptibility exist in different species and in different individuals of the same species.”
-----------+--------+---------------------------- | | Percentage Positive to Species | Number +-------+---------+---------- | Tested | 1 Per | 0.1 Per | 0.01 Per | | Cent | Cent | Cent -----------+--------+-------+---------+---------- Horse | 1 | 100 | 100 | 100 Dog | 91 | 83 | 35 | 0 Goat | 11 | 55 | 36 | 0 Rat | 10 | 30 | 20 | 0 Mouse | 7 | 70 | 14 | 0 Rabbit | 2 | 100 | 0 | 0 Guinea-pig | 12 | 33 | 0 | 0 Monkey | 9 | 22 | 0 | 0 -----------+--------+-------+---------+----------
The horse appears to be the most sensitive and the monkey and guinea-pig the most resistant species, while the dog would seem to have a sensitivity as near man as any of the species studied. No animal has yet been found which will give a blister from the application of mustard gas.
Smith, Clowes and Marshall[22] have studied the mechanism of absorption by the skin. They find that it is quite evident that the mustard gas is at first rapidly taken up by some element on, or adjacent to, the surface of the skin and for two to three minutes it may be completely removed, and for ten to fifteen minutes partially removed by prolonged washing With an organic solvent, and to a lesser extent with soap and water.
[Footnote 22: J. Pharmacol., =13=, 1 (1919).]
An interesting phenomenon is observed when the untreated normal skin of one subject is impressed for five minutes upon an area of skin of another subject, which has been exposed previously to the vapors of mustard gas. Under these circumstances both donor and recipient may develop burns (due to the transposition of the poison from one skin to another), the intensity of which will vary according to the circumstances and the respective sensitiveness of the participants. The degree of transposition is most strikingly observed in the intensity of the burn on the donor’s arm. If two similar exposures are made on the arm of a sensitive man, and one of these burns is treated, so to speak, by contact for five minutes, with the skin of a resistant man, the treated burn will be markedly less severe than the control, in some cases being entirely prevented. If, however, the recipient is equally sensitive to or more sensitive than the donor, the burns on the latter will exhibit far less difference. Both treatments may be effected at once, using two recipients, one more, and the other less, resistant than the donor. In such a case the burn brought into contact with the more resistant skin will be the less severe.
Similarly, if a sensitive individual impresses his arm alternately against burns of the same concentration and exposure on a resistant and sensitive man, the recipient receives a more severe burn from the sensitive than from the resistant man.
This indicates that the skin of a resistant individual exhibits a greater affinity or capacity for mustard gas than that of a sensitive one. There is an actual partition of the gas between the two skins, with an evident tendency to establish an equilibrium in which the larger portion of the gas will remain in that skin which possesses the greater capacity for it.
“A tentative explanation of this phenomenon can be made as follows. A three phase system is involved—the air over the skin surface constitutes the outer phase; some fatty or keratinous elements of the skin, the central phase; and a cellular portion of the skin the inner phase. The central phase is rich in lipoids and poor in water, while the inner phase is rich in water and poor in lipoids. After exposure to the vapors of dichloroethylsulphide the central phase is the absorbing agent and tends to establish equilibrium with the other two phases. On account of the lipoid nature of the central phase no damage is produced here because the compound is not hydrolyzed. On its passage from the central to the inner phase hydrolysis takes place within the cell and damage results when a sufficient concentration of hydrochloric acid is attained. The outer phase is constantly being freed from vapor by diffusion and convection currents, so more and more can evaporate from the central phase. The susceptibility of an individual depends on the relative power of the central phase to hold the poison in an inactive form (not hydrolyzed) and prevent its entry into the inner phase at a sufficient velocity to result in the formation of a toxic concentration. We do not attempt to localize the central or inner phases with any definite structure of the skin. As mustard is known to penetrate the sebaceous ducts the fat here might form one phase and the epithelial lining another.”
TACTICAL USE OF MUSTARD GAS
As before stated, mustard gas, like most other materials used in war, was discovered in peace. Indeed, Victor Meyer in 1886 worked out fairly completely its dangerous characteristics. Like phosgene and chlorine used before it, the materials for its production were available in considerable quantities through the manufacture of components either for dyes or photographic chemicals.
Mustard gas, besides being highly poisonous, has so many other important qualities as to have given it the designation during the war of the “king of gases.” That broad distinction it still holds. Its introduction at Ypres, on the night of July 12, 1917, changed completely the whole aspect of gas warfare and to a considerable extent the whole aspect of warfare of every kind. It is highly poisonous, being in that respect one of the most useful of all war gases. It produces no immediate discomfort. It has a considerable delay action. It burns the body inside or out, wherever there is moisture. Eyes, lungs and soft parts of the body are readily attacked. It lingers for two or three days in the warmest weather, while in cold, damp weather it is dangerous for a week or ten days, and in still colder weather may be dangerous for a month or longer whenever the weather warms up sufficient to volatilize the liquid. It is only slowly destroyed in the earth, making digging around shell holes dangerous for weeks and months and in some cases possibly a year or more.
The Germans first used it simply to get casualties and interfere with or break up the threatened heavy attacks by the British on the Ypres salient. While not stopping the inauguration of these attacks in the fall of 1917, the German use of mustard gas was so effective as to delay the beginning of those attacks for at least two weeks and thus gain valuable time for the Germans, besides causing serious casualties with consequent partial break up of companies, regiments and divisions in the English Army.
The German used his mustard gas throughout the fall of 1917 and the winter of 1917 and 1918, as above stated, to produce casualties, to destroy morale, to break up units, and to interfere with operations generally. During that time, however, he developed a more scientific use and when he started his big offensives in March, April, May and June, 1918, he used mustard gas _before_ the battles to cause losses, break up units and destroy morale, and also during the progress of battles to completely neutralize strong points which he felt he did not want to attempt to take by direct assault. Perhaps the most noted case of this was at Armentières in April, when he deluged the city to such an extent that mustard gas is said to have actually run in the streets. So effective was this gassing that not only did the British have to withdraw from the city but the Germans could not enter it for more than two weeks. It, however, enabled the Germans to take the city with practically no loss of life. There were numerous other cases on a smaller scale where mustard gas was used in the same way.
On account of its persistence it has been generally referred to as a defensive gas and for that purpose it is incomparable. The use of sufficient quantities of mustard gas will almost certainly stop the occupation of areas by the enemy and probably even stop his crossing them. It also enables strong points which it is not desired to attack to be completely neutralized,—that is, made so unhabitable that the area must be evacuated.
A use that was proposed toward the end of the war, and that will undoubtedly be made of the gas in the future, is to have it planted in drums in the ground and exploded when an enemy is attempting to advance. This would be a highly economic way to distribute great quantities of the material at the moment and in the place most heeded. It has even been proposed, and this would seem entirely feasible, to sprinkle certain of these areas with mustard gas by means of sprinklers attached to drums or even tanks mounted on trucks.
Just before the Armistice the German made another development in the use of mustard gas. Instead of the ordinary amount of explosive, which only fairly opened up the shell and allowed the liquid to escape, he filled nearly 30 per cent of the total space of the shell with high explosive. This completely broke up the shell and distributed the greater part of the liquid mustard gas in the form of a fine spray. This spray, when breathed, proved extremely deadly, as might be expected from the fact that when in the form of minute particles one can draw into the lungs in a single breath one hundred times or more the amount that he would get of pure gas.
Since mustard gas has such a delay action and is effective in such small concentrations it can be used very effectively in small calibre guns, as the 75 mm. or 3-inch. Furthermore, since it lasts for two or three days at the very least, a small number of guns can keep a very large area neutralized with the gas. With phosgene and similar non-persistent gases that volatilize almost completely upon the burst of the shell it is necessary to build up a high concentration immediately. The exact opposite is true of mustard gas. Mustard gas can be fired very slowly with the certain knowledge that all shells fired at one moment will be effective when the next is fired, though twelve hours or more may intervene between the first and last firing. Thus, while with phosgene a large number of guns are needed for a gas attack, with mustard gas the number can be reduced to one-tenth or even less. Mustard gas may be in the future and has been in the past used safely in hand grenades because of its very low vapor tension, whereby the pressure at ordinary temperatures is exceedingly low. This has an important bearing on cylinders and other containers for shipping mustard gas, that is, they need be only strong enough to be safe against handling and not to withstand the high pressure encountered with phosgene or chlorine cylinders.
In the future, mustard gas will be used in all the ways above stated and undoubtedly in many more. It can be fired in large quantities upon strong points to force their evacuation. It can be fired on the flank of attacking armies for protection against counter-attacks. It can be fired against the enemy artillery at all times to silence them and stop their firing. It was thus used by the Americans in the Argonne against the enemy on the east bank of the Meuse River, this river separating the American and German armies. It was extremely effective in stopping the enemy’s artillery. The high explosive mustard gas shell, not only because of its persistency but because of its quick deadliness, can be fired singly and be depended upon to do its work wherever there be men or animals. One of the greatest uses will be by simple sprinkling from aeroplanes.
The future will see mustard gas used at nearly all times with a certain quantity of a powerful lachrymator or tear gas. This is for the reasons, as stated in the beginning, that mustard gas causes no immediate discomfort and has no objectionable smell. Accordingly, if the battle be critical, men may continue to fight from four to eight hours in a mustard gas atmosphere without masks. It is true the casualties will be high with a high death rate. Nevertheless, this period of time might enable the artillery to do such effective work as to completely stop an attack. If, however, at the instant mustard gas firing is begun a number of powerful lachrymatory shells are sent over, the immediate wearing of the mask is forced. The enemy is then subject to all the burning effects of mustard gas as well as the discomfort of long wearing of the mask.
It may confidently be expected that further developments in the use of mustard gas will be made, as well as further developments in methods of throwing it upon the enemy or of bursting shell containing it in his midst.
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