CHAPTER XVII
TOXIC SMOKES
The introduction of diphenylchloroarsine as a poison gas really introduced the question of toxic smokes. This material, as has already been pointed out, is a solid, melting at about 30°. In order to secure efficient distribution, the material was mixed with a considerable amount of high explosive. When the shell burst, the diphenylchloroarsine was finely divided or atomized and produced a cloud of toxic particles. Since smoke particles are only slightly removed by the ordinary mask, this formed a very effective means of chemical warfare.
An analogous result was obtained by the use of poison gases, such as chloropicrin, in a smoke cloud produced from silicon or stannic chloride. Here, however, the toxic material was a real gas, and so the real result attained consisted in forcing the men to wear their masks in all kinds of clouds. The true toxic smoke went further in that the ordinary mask offered little protection and thus compelled the warring nations to develop a special type of smoke filter.
These smoke clouds consist of very small particles, which may be considered as a _dispersed_ phase, distributed in the air, which we may call the _dispersing medium_. The dispersed phase may be produced by _mechanical_, _thermal_, or _chemical_ methods.
_Mechanical dispersion_ consists in the tearing apart of the material into a fine state of subdivision. It may be called a hammer and anvil
## action. The more powerful the mechanical force, the smaller the
resulting particles. This may be accomplished by the use of a high explosive, such as the Germans used in the case of diphenylchloroarsine.
The production of smoke by _thermal dispersion_ depends essentially upon the fact that when a substance of sufficiently low vapor pressure is volatilized, and the vapors are passed into the air, they recondense on the nuclei of the air to form a smoke. Vaporization from an open container, permitting the vapors to pass directly to the air without being quickly carried away from the surface of evaporation, produces smoke having larger particles, because each particle formed remains for an appreciable period of time in contact with air saturated with vapor, and hence grows very rapidly.
The easiest way to produce small smoke particles is to mix the toxic material directly with some fuel which will produce a large amount of heat and gas upon burning. When this mixture is enclosed in a container having a small orifice, upon burning, the toxic vapor and gas will pass through this orifice at high velocity; it has been demonstrated by Lord Rayleigh that the size of the particles depends upon the velocity of emission of the gas from a given orifice.
The product of _chemical combination_ may include a super-saturated vapor, which condenses into small particles.
_Explosive dispersion_ is really a combination of _mechanical dispersion_ followed by _thermal dispersion_.
PENETRATION
The fundamental idea underlying all the work on toxic smokes is to obtain a smoke that has marked penetrating power. Screening power is not important here. In addition to penetration, a smoke should be highly toxic and have a slow rate of settling.
Penetration may be tested by the use of a standard filter; a suitable filter for this purpose is one which does not remove the smoke to such an extent that measurement of its concentration becomes difficult, and one which does not become clogged quickly by the smoke. A filter consisting of two pads of felt, placed side by side and arranged so that the smoke first comes in contact with the thinner and less dense pad has been found very satisfactory.
In testing penetration, smoke is produced by dispersing one gram of the toxic substance in a sheet iron box of 1000 liters capacity. After 5 minutes a steady concentration is usually attained and the smoke is then forced through a Tyndall meter, (see page 299) after dilution with air, where the initial concentration is determined. It then passes through the standard filter, and through a second Tyndall meter, where the final concentration is measured. The difference of the two readings gives the amount of smoke retained by the filter. The penetration is ordinarily represented by a series of figures, which decrease from a maximum value at the beginning of the test to a minimum at a point where the filter permits the passage of so little smoke that it cannot be measured. This decrease is due to decrease in penetrating power and concentration of the smoke, and to increase in filtering power of the filter as a result of plugging. Usually five degrees of penetration are recognized, excellent, good, fair, poor and very poor.
[Illustration: FIG. 100.—Penetration Apparatus Used to Test Toxic Smokes.]
A portable penetration apparatus is shown in Fig. 100. In using the apparatus, the smoke producing material is so placed with reference to the apparatus that the sample is taken about 20 feet down the wind, so that the smoke is appreciably diluted. One man is stationed at each Tyndall meter and takes readings as fast as his recorder can write them, so that the smoke density, before and after the filter, can be followed very closely.
PHYSIOLOGICAL ACTION
In addition to a high penetrating power a smoke should also possess great toxic, irritant, sternutatory, or lachrymatory power. These properties are tested by exposing mice to the smoke in the chamber. They are placed in the chamber at the beginning of the run, and exposed for 10 minutes to the smoke from 1 gram of the material. While these tests are only qualitative in character, they give a fairly good notion of the relative value of different materials.
QUANTITATIVE RELATIONSHIPS
It has been found that, if the optical readings from the Tyndall meter are plotted as ordinates against the time _t_ (the time elapsed after detonation) as abcissas, and that portion of the curve between _t_ = 0 and _t_ = 30 considered, the curve generally descends sharply at first, from a high point representing the density immediately after the production of the smoke, to a point in the neighborhood of _t_ = 8, where it flattens out and descends much more slowly with a slope that changes little. The area under the significant portion of the curve, that is, the area circumscribed by the curve from the point _t_₃₀ to _t_₀, the vertical axis from this point to the origin, the horizontal axis from the origin to _t_₃₀ and the line perpendicular to this axis, cutting the curve at _t_₃₀, is a rough measure of the relative values of different smokes. This area is calculated as the sum of two rectangles, from _t_₀ to _t_₈ and from _t_₈ to _t_₃₀.
Some results are as follows:
Area 30 Phenyldichloroarsine 181 Triphenyldichloroarsine 178 Diphenylcyanoarsine 137 Diphenylchloroarsine 101 Cyanogen bromide 94 Methyl dichloroarsine 70 Phenylimidophosgene 69 Mustard gas 38
The curves in Fig. 101 show the way in which the readings fall off with time. Each substance of course has its characteristic curves.
[Illustration: FIG. 101.—Typical Curves Showing the Decrease in Concentration of Smoke Cloud with Time.]
TOXIC MATERIALS
The selection of materials for the production of toxic smokes can only be carried out experimentally. A number of very toxic substances have been shown to be valueless as toxic smokes because of low penetration, decomposition during the process of smoke production, or for other reasons.
Arsenic compounds produce smokes distinctly better than the average. Inorganic compounds which have high melting and boiling points are very poor smoke producers. The only exception to this is magnesium arsenide, which may suffer decomposition. Compounds like mercuric chloride and arsenic tribromide, which boil or sublime at comparatively low temperatures, produce good smokes. Most materials which boil below 130° C. produce no smoke as they evaporate on dispersion. It is difficult to set any upper limit for the boiling point beyond which materials do not produce good smokes, but in all probability 500° C. is not far from the maximum. Liquids and solids are, on the whole, almost equally good as smoke producers. The physical condition of the material has no great effect upon the amount of smoke which it will produce. This seems to depend only upon the physical and chemical properties of the material.
TOXIC SMOKE APPARATUS
It has been mentioned above that the Germans used a shell, containing solid diphenylchloroarsine and a high explosive. A 10.5 cm. shell (Blue Cross) was about two-thirds filled with cast trinitrotoluene and contained a glass bottle with 300-400 grams of toxic material. Diphenylchloroarsine was also used in shell, in solution, a mixture of phosgene and diphosgene (superpalite) being the ordinary solvent (Green Cross). Mixtures of diphenylchloroarsine and phenyldichloroarsine were also used.
In the case of high explosive shell, the use of a separate container appears to be desirable, because a mixture with the explosive seriously decreases its sensitiveness and even its destructive power. There is also a question as to the stability of such a mixture. However, 75 mm. shell containing 30 per cent diphenylchloroarsine mixed with T. N. T. gave good clouds of toxic smoke.
TOXIC SMOKE CANDLE
Two toxic smoke candles were developed by the Chemical Warfare Service, known as the B-M Toxic Smoke Candle, perfected by the Pyrotechnic Section of the Research Division, and the Dispersoid Smoke Candle, developed by the Dispersoid Section.
The B-M Toxic Smoke Candle consists of a bottle-shaped sheet steel toxic container set into a can, containing smoke mixture. The heat from the burning mixture causes the distillation of the toxic material. The toxic vapor is discharged through a nipple, screwed into the neck of the container and extending over the top of the smoke can. Steel wool is used in the toxic container to reduce the violent boiling and spattering of the material. A small amount of steel wool, held in place by a wire screen, is also used in the nipple for the same purpose. The toxic container is sealed by a fusible metal plug, melting at 90° C., cast into a retainer at the base of the nipple. The fusible plug melts upon the first application of heat and allows free passage of the vapor into the smoke cloud. The ignition of the apparatus is effected by means of a simple match head and an accompanying scratcher.
[Illustration: FIG. 102.—Toxic Smoke Cloud from 500 D. M. Candles.]
The candles were placed in 5 parallel rows which were 2 yards apart, each row containing 100 candles on a 100 yard front. The total time of active smoke emission was 23 minutes.
The first evolution of smoke occurs about 10 seconds after the first appearance of flame. About one minute after ignition the toxic material will begin to distill into the smoke cloud and this will continue for about four minutes. The burning of the candle should be complete in about six minutes.
[Illustration: Dispersoid Candle British Candle
FIG. 103.—Comparison of Dispersoid and British D. M. Candles.]
The Dispersoid Toxic Smoke Candle differs from the B-M candle in that the toxic container is not used. A mixture of smokeless powder and the toxic material (diphenylchloroarsine or D. M., an arsenical obtained from arsenic trichloride and diphenylamine) is filled directly into the container, a cylindrical can 3.5 inches in diameter and 9 inches high made from 27 gauge sheet metal, and packed under a total pressure of 2,500 pounds. The top of the candle is a metal cover, containing the match head scratcher, which is separated from the match head by a Manila paper disc. These are the same as those used in the B-M candle. The candle has a total weight of about 4.25 pounds, of which 3.6 pounds are the smoke mixture, containing about 1.3 pounds of toxic material.
In operating the candles, the cover is removed and the match head ignited by friction with the scratcher. The match head burns through the cardboard and ignites the powder. The heat and gas produced by the combustion of the powder vaporizes the particles of the toxic material and carries the vapors out through the orifices at a high velocity whereupon they recondense to form a smoke. The rapid emission of the vapors through the orifice prevents any possibility of their ignition.
The time before good emission of smoke takes place after the ignition of the match tip of a candle is 30 seconds. The average time of vigorous smoke emission is from four to five minutes. The result of a field test with the dispersoid candle is shown in Fig. 102. A comparison of a British and a Dispersoid candle is shown in Fig. 103. It should be stated that this may not have been a fair test as only one British candle was available for the comparative test.
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