CHAPTER XVI

SCREENING SMOKES

The intelligent use of screening smokes in modern infantry tactics offers innumerable advantages through concealment and deception. It confers upon daylight operations many of the advantages which were gained by conducting operations at night with few of the disadvantages of the latter.

Smoke screens have been frequently used by the Navy and by Merchantmen; a common method of escape was to shut off the air from the fire with consequent incomplete combustion of the fuel, thus causing a cloud of dense black smoke. This is often mentioned in the blockade runners of the days in the Civil War, where wood, high in pitch and rosin, was freely introduced into the furnaces, in order that they might escape under cover of this smoke.

Early in the present war it was found that black smoke had a low obscuring power, showed frequent rents or holes and were difficult to standardize. Their production also caused a considerable loss in the speed of the vessel. They therefore fell into disuse except for emergency purposes and today the standard smoke for screening purposes of all kinds is, without exception, white.[33]

[Footnote 33: While it is a well known fact that black smoke is not as efficient as white smoke for screening purposes, the reason for this fact is not clear.]

PROPERTIES OF SMOKE CLOUD

The properties most desired in a screening smoke, apart from low cost, are: (_a_) _Maximum screening power_, which refers to the question of density, i.e., a relatively thin layer must completely obscure any object behind it, and (_b_) _Stability_, which implies, among other things, a low rate of settling or dissipation. There is little reason to doubt that, within limits, the smaller the particles of a smoke cloud, the more completely will the smoke possess these qualities. The screening power of a smoke cloud depends very largely upon the scattering of the light coming through it, and by analogy with those peculiar solutions which we call colloidal, we should expect the scattering to increase as the degree of subdivision increases, within limits. The rate of settling is unquestionably an inverse function of the size of the particles. The chief aim, therefore, in smoke production is to attain as high a degree of subdivision as possible. Methods may be classified as good or bad, in so far as they satisfy or fail to satisfy this criterion.

RAW MATERIALS FOR SMOKE CLOUDS

It is obvious that only gases or substances capable of being brought into the vapor state or into a very fine state of subdivision can be used for producing smoke clouds. The reaction product, of which the smoke particles consist, should preferably be:

(_a_) _Solid._ Otherwise the particles will tend to grow in size by condensation of the liquid particles present in the cloud.

(_b_) _Non-volatile._ If volatile, the particles will disappear by evaporation as the cloud is diluted by air currents. Larger particles will also form at the expense of the smaller ones.

(_c_) _Non-deliquescent._ If the particles are deliquescent, they will tend to grow by condensation of water vapor upon them.

(_d_) _Stable_ towards the usual components of the atmosphere, especially moisture.

While it might seem that it would be difficult to fulfill these conditions, there are several chemical compounds which have been successfully used as smoke producers. This does not mean that they fulfill all the conditions, but they represent a compromise between the various requirements.

=Phosphorus.= One of the earliest materials to be used in smoke clouds was phosphorus. This is prepared on a commercial scale by heating phosphate rock (which contains calcium phosphate) with sand and coke in an electric furnace. Phosphorus occurs in two forms, white and red. _White phosphorus_, which is formed when the vapor of the substance is quickly cooled, is, in the pure state, almost colorless, melts at 44° C., boils at 287° C., is readily soluble in various solvents, and is luminous in the air, at the same time emitting fumes (the oxidation product, phosphorus pentoxide). On gentle warming in the air, it takes fire and burns with a brightly luminous flame. _Red phosphorus_ is obtained by heating white phosphorus out of contact with the air, to a temperature of 250° to 300° C. Red crusts then separate out from the colorless liquid phosphorus, and almost the entire amount is gradually converted into a red, solid mass. If this is freed by suitable solvents from the small amounts of unchanged white phosphorus, a dark red powder is obtained, which remains unchanged for a long time in the air, does not appreciably dissolve in the solvents for white phosphorus, does not become luminous, and can be heated to a fairly high temperature without igniting. Further, red phosphorus is not poisonous, while white phosphorus is highly so.

Either form burns to phosphorus pentoxide, which is converted by the moisture of the air to phosphoric acid,

4P + 5O₂ = 2 P₂O₅ 2P₂O₅ + 6H₂O = 4H₃PO₄

Since one pound of phosphorus takes up 1.33 pounds of oxygen and 0.9 pound of water, it is not surprising that phosphorus is one of the best smoke producers per pound of material. Comparison of the value of the two forms for shell purposes have invariably pointed to the superiority of the white variety.

In addition to its use as a smoke producer, it is used in incendiary shell and in tracer bullets. For incendiary purposes a mixture of red and white phosphorus is superior.

=Chlorosulfonic Acid.= Chlorosulfonic acid, ClSO₂OH, was first employed by the Germans to produce white clouds, both on land and on sea. For this purpose, they sprayed or dropped it onto quicklime, the reaction between it and the lime furnishing the heat necessary for volatilization, though in this way about 30 per cent of the acid is wasted.

Chlorosulfonic acid is obtained from sulfur trioxide and hydrogen chloride, which combine when gently heated:

SO₃ + HCl = ClSO₂OH

[Illustration: FIG. 86.—75 mm. White Phosphorus Shell. 2 seconds after bursting.]

On a commercial scale, hydrogen chloride is passed into 20 per cent oleum, until saturation is reached. This is heated in a nitric acid still, when the chlorosulfonic acid distills over between 150°-160° C. With 30 per cent oleum, the conversion factor is about 42 per cent. The residue in the still is about 98 per cent sulfuric acid.

It forms a colorless liquid, boiling at 152° C., and having a density of 1.7.

Chlorosulfonic acid fumes in the air, because reaction with water forms sulfuric acid and hydrochloric acid.

ClSO₂OH + H₂O = H₂SO₄ + HCl

This material was not used by the United States since oleum was found superior.

=Oleum.= Oleum is a solution of 20 to 30 per cent sulfur trioxide (SO₃) in concentrated sulfuric acid. It has been used by the Germans to produce clouds on land and sea, by its contact with quicklime, and by the Americans for screening tanks and aeroplanes. Sulfur trioxide has been found to be superior as a shell filling. It is believed that the smoke producing power of oleum is due solely to its sulfur trioxide content, the sulfuric acid itself acting only as a solvent. The rather high freezing point of the oleum containing high percentages of sulfur trioxide is a disadvantage.

=Sulfur Trioxide.= Sulfur trioxide, SO₃, is a colorless mobile liquid, which boils at 46° C. and solidifies to a transparent ice-like mass, melting at 15° C. It is prepared by passing a mixture of sulfur dioxide and oxygen over finely divided platinum or other catalysts at a temperature between 400 and 450° C. Sulfur trioxide can only be used as a filler for shell and bombs, and is probably the best substitute for phosphorus.

=Tin Tetrachloride.= Tin tetrachloride, SnCl₄, is obtained by the

## action of chlorine on metallic tin. It is a liquid, boiling at 114° C.,

and having a density of 2.2. It fumes in the air, because it hydrolyzes to stannic hydroxide:

SnCl₄ + H₂O = Sn(OH)₄ + 4 HCl

It makes a better and more irritating smoke for shell and hand grenades, than either silicon or titanium tetrachlorides. Since there is practically no tin in this country, the other tetrachlorides were developed as substitutes.

=Silicon Tetrachloride.= Silicon tetrachloride, SiCl₄, is prepared from silicon or from impure silicon carbide by heating it with chlorine in an electric furnace. The raw material (silicon carbide) is a by-product in the manufacture of carborundum. It is a colorless liquid, boiling at about 58° C., and fumes in moist air, owing to hydrolysis:

SiCl₄ + 4 H₂O = Si(OH)₄ + 4 HCl

It is not very valuable in shell, though it is more effective on moist, cool days than on warm, dry ones. Its greatest use is found in the smoke cylinder, combined with ammonia. By the action of the moisture of the air, the following reaction takes place:

SiCl₄ + 4 NH₃ + 4 H₂O = Si(OH)₄ + 4NH₄Cl

The addition of a lachrymator gives a mixture which works well in hand grenades for mopping up trenches.

=Titanium Tetrachloride.= Titanium tetrachloride, TiCl₄, is made from rutile, TiO₂, by mixing with 30 per cent carbon and heating in an electric furnace. A carbonitride is formed, which is said to have the composition Ti₅C₄N₄, but the actual composition may vary from this to the carbide TiC. This product is heated to 600-650° C., and chlorine passed through, giving the tetrachloride. It is a colorless, highly refractive liquid, which boils at about 136° C., is stable in dry air and fumes in moist air. The best smoke is produced by using 5 parts of water to one of the tetrachloride, instead of the theoretical 4 parts [which would form Ti(OH)₄.] Since it is more expensive to manufacture and not as effective as silicon or tin tetrachloride, it is used only as an emergency material.

=Berger Mixture.= One of the most important smoke materials was the zinc-containing mixture, which was used in the smoke box, the smoke candle, certain of the smoke grenades and in various forms of colored smokes. The basis of this was the _Berger Mixture_, which had the composition:

Zinc 25 Carbon tetrachloride 50 Zinc oxide 20 Kieselguhr 5

This formula produced a light gray carbon smoke, with much carbon in the residue. In this mixture the zinc and carbon tetrachloride react to form zinc chloride and carbon; the kieselguhr keeps the mixture solid by absorbing the tetrachloride, while the zinc oxide is practically useless, as its absorbing power is small.

In order to accelerate the reaction and to oxidize the carbon, thereby changing the color of the smoke from gray to white, an oxidizing agent was added. Sodium chlorate was chosen for economic reasons. The reaction now proved to be too violent, and the zinc oxide was replaced by ammonium chloride. This cooled the smoke, retarded the rate of burning and added to the density of the smoke, since the obscuring power of the ammonium chloride is high. The kieselguhr was replaced by precipitated magnesium carbonate, which is as good an absorbent, gives a much smoother burning mixture, and also adds somewhat to the density of the smoke by virtue of the magnesium mechanically expelled. The mixture then had the composition:

Zinc 34.6 Carbon tetrachloride 40.8 Sodium chlorate 9.3 Ammonium chloride 7.0 Magnesium carbonate 8.3

SIZE OF SMOKE PARTICLES

In the problem of smoke production, the size of the particle is of great importance. Being a physical quantity it can easily be correlated with such physical properties as settling, diffusion, coagulation, and evaporation. These factors are more important in connection with toxic smokes, since there the penetration factor must be considered.

Smoke appears to consist of particles of all sizes from 10⁻³ cm., which may just be resolved by the unaided eye, to molecular dimensions, 10⁻⁸ cm. The larger particles settle out most rapidly and so do not remain long in suspension.

MEASUREMENT

Wells and Gerke have developed a form of ultra-microscope which is well adapted to the measurement of the size of smoke particles. The ultra-microscope is a low power microscope using intense dark ground illumination for viewing particles which are too small to be seen by transmitted light. They are rendered visible in this way, since any object, no matter how small, which emits enough light to affect the retina is visible, provided the background is sufficiently dark. Thus stars are visible at night and dust particles are easily seen in a sunbeam in a darkened room. The larger particles, viewed in this way, do not appear larger but brighter. The apparent size of the particles is determined by the diffraction pattern and is thus dependent only on the optical system used to view them. The more intense the incident light, the brighter the particles appear. In the ultra-microscope described, the image of an intense source, such as a concentrated filament lamp, or an arc, is focused upon the particles in the microscopic field, but the axis of the illuminating beam, instead of coinciding with the axis of the microscope, as ordinarily used, is perpendicular to it. The beam itself, therefore, never enters the microscope at all, but passes under the objective into a blackened chamber where it is absorbed. The field of the microscope is made dark by placing underneath the objective another “black hole” or blackened chamber with an opening just a little larger than the field.[34]

[Footnote 34: This ultra-microscope is described in _J. Am. Chem. Soc._ =41=, 312 (1919).]

The method used for measuring the velocity consisted in causing the

## particle to describe a definite stroke many times in succession in an

electric field. This was accomplished by reversing the direction of the field with a rotating commutator. The convection due to the source of light is perpendicular to this motion so that a zigzag line is obtained (see Fig. 88). The amplitude of this oscillation is an accurate measure of the distance traversed by the particle under the electric force for a definite small interval of time. The speed of the rotating commutator and the electric field are both susceptible of precise measurement, so that the size of a single particle is precisely determined.

[Illustration: FIG. 87.—Ultramicroscope for Measuring Size of Smoke

## Particles.]

[Illustration: FIG. 88.—Measurement of Smoke Particles by Use of Ultramicroscope.]

When a sample of smoke is viewed in the ultra-microscope, it appears like the starry heavens, except that the stars are dancing violently about. At first little distinction is made between the particles, as there seems to be no order in their motion, but soon it becomes evident that the brighter particles are more sluggish than the dim ones. This is due to the greater mass of the bright particles, for they are larger. The particles are all moving slowly away from the source of light and eventually diffuse to the walls of the cell.

When the electric field is turned on, about one-third of the particles immediately migrate, about equally in both directions, to the two electrodes. If the field is reversed, the direction of migration is reversed and if the commutator is used the particles oscillate regularly. Sometimes the particles may be seen to combine and become neutral, in which case oscillation ceases.

CONCENTRATION OF SMOKE

In measuring the concentration of smokes, the following terms are useful:

=Density.= The density of a smoke is defined as the reciprocal of the thickness of the smoke layer in feet necessary to obscure a given filament. Thus six inches of a smoke of density 2.0 is required to obscure an electric light filament, whereas one requiring four feet would have a density of 4. Another way to show the significance of this definition is to point out that if a definite weight of a stable smoke is diluted with air after it is formed, the product of the volume by the density always remains constant. Any marked variation in this rule may be taken as evidence that the particles of smoke are undergoing a change, in most cases due to evaporation.

=Total Obscuring Power.= The volume of smoke produced per unit weight of material used is the second factor in determining the value of a smoke. The product of this volume per unit weight by the density of the smoke is the real measure of effectiveness, and is called the total obscuring power (T. O. P.) of the smoke. If the volume is expressed in cubic feet per pound and the density in reciprocal feet, the unit of T. O. P. is square feet per pound. That is, it expresses the square feet of a smoke wall, thick enough to completely obscure a light filament behind it, which could be produced from a pound of the reacting substances. The total obscuring power of some typical smokes are as follows:

Phosphorus 4600 NH₄Cl(NH₃ + HCl) 2500 SnCl₄ + NH₃ + H₂O 1590 Berger Mixture 1250 SnCl₄ + NH₃ 900 SO₂ + NH₃ 375

In all measurements of density, and therefore of T. O. P., the _rate of burning_ must be considered. If a slow burning material be compared with a rapid one, the former will not reach its true maximum density, as a great deal of the smoke may settle out during the time of burning. Comparisons of T. O. P. are significant only when made on smoke mixtures of the same type and in about the same quantities.

MEASUREMENT

Two methods of measuring the effectiveness of a smoke cloud have been devised, one, the smoke box, which measures the obscuring power directly by observing at what distance a lamp filament is obscured by intervening smoke, the other, the Tyndall meter, which measures the intensity of the scattering of the light.

The earliest measurements of smoke intensity are perhaps those of Ringelmann (_Revue Technique_, =19=, 286), who devised the well known chart of that name, intended mainly for measuring intensities of black smoke issuing from a chimney at a distance. The first measurements for military purposes are probably due to Bertrand, who made numerous comparative studies with his “salle opacimetrique.” This was a room 23 × 14 × 3.6 meters, with 7 windows. Two doors, one provided with 3 oculars 2 cm. in diameter, gave access to the room. On the other door, opposite the first, were hung several black signs. Six pairs of columns were spaced along the room at measured distances. When a smoke is produced in the room, the black paper signs first become invisible, then the door itself, and finally the columns, pair by pair. They reappear in the reverse order, and as a measure of relative opacity Bertrand took the time elapsing between the detonation and the reappearance of the farther door.

[Illustration: FIG. 89.—Tyndall Meter.]

[Illustration: FIG. 90.—Cottrell Precipitation Tube.]

=Smoke Box.= The smoke box, used by the C. W. S., was constructed of wood with tight joints, and had a moveable brass rod running through it to which was attached a small size 25-Watt Mazda lamp. The density of each smoke introduced in the box is determined by moving this lamp back and forth until a point is reached when the pattern of the filament can just be distinguished by the observer looking in at the glass window, external light being excluded by a black cloth. The thickness of the smoke layer between the glass window and the light is recorded as the measure of the smoke density. For field tests, a larger box, 6 × 8 × 8 feet (288 cubic feet) was constructed. The observation light was moveable in a line connecting the mid-points of opposite sides of the box. To insure uniform distribution of smoke, a fan with 18-inch blade revolved at any desired speed between 60 and 250 r.p.m. With this, results are obtained indicating both the original density and its stability.

=Tyndall Meter.= The Tyndall meter was first devised for studying smokes and mists. Tolman and Vliet adapted it to Chemical Warfare purposes, and used it in studying the properties of smokes.

The apparatus (Fig. 89) consists eventually of an electric light bulb, a condensing lens giving a beam of parallel light which passes through the diaphragm, and a Macbeth illuminometer for measuring the strength of the Tyndall beam. In case the material is a liquid suspension or solution, it is introduced into a cylindrical glass tube, while smokes and mists are premixed directly through the apparatus. The long closed tubes are provided, respectively, for absorbing the beam after it has passed through the disperse system and for giving a dark background for observing the Tyndall beam. Methods of standardization are given in the _Journal of the American Chemical Society_, =41=, 299.

A third method for analyzing smokes consists in the use of an electrical precipitator. This apparatus consists essentially of a modified Cottrell Precipitator, with a central wire as cathode surrounded by a cylindrical foil as anode (Fig. 90). The smoke to be analyzed is drawn through the apparatus at a known rate, and the

## particles of smoke precipitated on the foil by means of a high voltage,

direct current. The determination of concentration is made by weighing the foil before and after precipitation.

APPARATUS FOR SMOKE PRODUCTION

SMOKE BOX

The smoke box was developed for the Navy for use when it was desirable to have the smoke screen generated away from the ship. (The smoke funnel, described later, was operated on board ship). The float consists of an iron container (holding the smoke mixture) surrounded by an iron float to support the apparatus when it is thrown into the water (Fig. 91). The iron container consists of a cylinder 22 inches high and 10 inches in diameter. One inch holes are bored 1½ inches from the top of this cylinder, from which the smoke is emitted. The iron float is about 2 feet in diameter and 8 inches deep. This box holds approximately 100 pounds of smoke mixture, and is so constructed that it will float one hour. When ignited, the mixture burns 9 to 9½ minutes. The smoke produced has a T. O. P. of about 1900. Fig. 92 shows the Navy Smoke Box in action.

[Illustration: FIG. 91.—Navy Smoke Box.]

[Illustration: FIG. 92.—Navy Smoke Box in Action.]

SMOKE CANDLE

Smoke candles are used for producing a cloud of smoke for screening purposes in or behind the lines. They are made by packing about three pounds of the modified Berger Mixture in a container (Fig. 93) (galvanized can 5¼ inches by 3½ inches) and are lighted by means of the match head type of ignition. Smoke is given off at a uniform rate for about 4 minutes, forming a dense, fog-like cloud which hangs low (Fig. 94). This smoke is absolutely harmless, and can be breathed without discomfort. The obscuring power is high and, with a favorable wind, a small number of the candles will produce a screen sufficiently dense to allow operations to be carried out unseen by the enemy.

[Illustration: FIG. 93.—B. M. Smoke Candle.]

SMOKE GRENADE

The smoke grenade is also designed for use in trench and field warfare, where it is desired to produce a dense smoke screen. It is made by packing 340 grams of the standard smoke mixture in an ordinary light metal gas grenade. Around the top of the grenade are vents closed by a zinc strip. The ignition is caused by the standard bouchon when the grenade is thrown. The heat of the reaction burns through the zinc strip and a dense cloud of smoke is evolved for 45 seconds.

[Illustration: FIG. 94.—Smoke Cloud from B. M. Candle.]

Stannic chloride has also been used extensively in hand grenades, as it gives a very disagreeable cloud of smoke upon detonation. Due to the high prices and urgent need of tin for other purposes, silicon tetrachloride was substituted for tin tetrachloride towards the close of the war. A mixture of silicon tetrachloride and chloropicrin was also used. This not only gives a very good smoke cloud, but combines with it the toxic properties of the chloropicrin cloud.

The method of firing the smoke grenade is the same as that of any grenade using the same type of bouchon. Usually the grenade is grasped in the hand for throwing in such a manner that the handle of the bouchon is under the fingers. The safety clip is pulled out with the other hand and the grenade is thrown with an overhand motion. When the grenade leaves the hand, the handle of the bouchon flies off, allowing the trigger to hit the cap which ignites the fuse.

The white phosphorus combined hand and rifle grenade became the standard smoke grenade by the end of the war. Stannic chloride was used to clear out dugouts, but not as a smoke producer.

STOKES’ SMOKE SHELL

[Illustration: FIG. 95.—Stokes’ Smoke Shell.]

The Stokes’ smoke shell was perfected to furnish a means of maintaining the best possible smoke screen at long ranges by means of an easily portable gun. The 3-inch Stokes shell, as adapted for combustion smokes, weighs about 13 pounds and contains about 4 pounds of standard smoke mixture. This shell is designed to produce a continuous screen over a period of 3 to 4 minutes.

LIVENS SMOKE DRUM

The Livens smoke drum was designed for use with the 8-inch Livens projector, so as to produce a smoke screen of large volume and long duration at long ranges. The drum, as adapted for combustion smokes, weighs 17.5 pounds empty and 49 pounds loaded. The smoke-gas mixture was specially adapted for use in the Livens drum.

[Illustration: FIG. 96.—Livens Smoke Bomb.]

Smoke mixtures in Livens were never used to any considerable extent in the war and it is questionable if they ever will be. A Livens can usually only be fired once before resetting, hence Stokes mortars are used whenever possible.

SMOKE FUNNEL

The smoke funnel was developed for the production of a white smoke cloud from the stern of a vessel. The smoke producing materials are liquid ammonia and silicon tetrachloride, with carbon dioxide as a compressing medium. This is the most satisfactory compressing medium, because: (1) The silicon tetrachloride is forced out at nearly constant pressure. (2) The carbon dioxide is easily compressed to a liquid and can be handled in this form. Further, it has a vapor pressure of 800 pounds at 60° F., and a cylinder can be nearly emptied without loss in efficiency. (3) Carbon dioxide is sufficiently soluble in silicon tetrachloride to cause the latter to effervesce and thus materially aid in its evaporation on spraying. (4) Liquid carbon dioxide, behaving in a manner similar to liquid ammonia, affords a means for the silicon tetrachloride to “keep pace” with the ammonia, under changes in temperature, and thus ensures a more nearly neutral, and therefore the most effective, smoke.

[Illustration: FIG. 97.—Navy Smoke Funnel.]

The smoke funnel proper consists of an open end cylinder, about 2 feet in diameter and 7 feet long, mounted in a horizontal position on an angle iron frame. At one end is an 18-inch fan securely fastened to the cross supports. This fan is operated by hand, through gears giving a ratio of about 30 to 1. The silicon tetrachloride enters the cylinder through a pipe, which terminates in four spray nozzles, while the ammonia enters through a single nozzle. The air forced into the funnel serves to hydrolyze the silicon tetrachloride and mixes the vapors. The resulting reaction evolves a dense white cloud of very large volume and high obscuring power. One set of cylinders is capable of maintaining this cloud for over 30 minutes. Under normal conditions the discharge is at the rate of 2 pounds of silicon tetrachloride to 1 pound of ammonia. To stop the smoke, the silicon tetrachloride is closed first, the ammonia allowed to run about half a minute, and the fan is shut off last.

[Illustration: FIG. 98.—Navy Smoke Funnel in Operation.]

SMOKE KNAPSACK

The smoke knapsack furnishes a portable apparatus for smoke production. The gross weight is about 70 pounds; when in operation it gives a dense white smoke for about 15 minutes. The operation may be intermittent or continuous and the quantity of smoke is sufficient to completely hide one platoon of men in skirmish formation with a 5-mile per hour enfilade wind. The apparatus consists of two steel tanks about 26 inches in height and 6 inches in diameter. From the side of each tank, but near the bottom, extends a short pipe on which is placed a suitable valve. A flexible armored hose connects the valve to a short length of pipe which is equipped with spray nozzle. The cylinders are charged with silicon tetrachloride and ammonia under pressure. The valves may be operated with the left hand, while the sprayer apparatus is held in the right. The release buckles are within easy reach of both hands.

SHELL

While many special devices have been developed by means of which the gas troops and infantry are able to set up smoke clouds on short notice, the smoke shell, fired by the artillery, always played an important part in this work. In the same way that a large number of the poison gases were adapted to artillery use, so were most of the smoke producing substances.

As a filler for smoke shell, phosphorus easily ranks first, and is approached only by sulfur trioxide in very humid weather. A rough approximation to the relative values of some of its rivals is given in the following table:

White phosphorus 100 Sulfur trioxide 60-75 Stannic chloride 40 Titanium chloride 25-35 Arsenic chloride 10

Comparison of the value of different forms of phosphorus for shell purposes has invariably pointed to the superiority of the white variety. Mixtures of white and red (2 to 1) have also proved effective.

A complete barrage over a front of 200 yards can be established in from 40 seconds to 1 minute and maintained by firing a salvo followed by battery fire of 3 seconds. Four 4.5-inch howitzers could maintain an effective barrage over a front of 1000 yards. The influence of sunshine is very marked, as in moist, cool weather one shell every 15 seconds is sufficient.

[Illustration: FIG. 99.—Smoke Screen for Tanks.]

SCREENING TANKS

Tests have demonstrated (see Fig. 99) that successful smoke screens for tanks may be produced by spraying oleum into the exhaust. On a 7-ton tank of the Renault type (40 H. P.) 110 cc. per minute produced a large volume of smoke, which had excellent covering power, and which could be made intermittent or continuous at will.

The same method may be applied to aeroplanes, and to ships. It is calculated that a cylinder containing 300 pounds of 20 per cent oleum will maintain a smoke screen on a ship for a period of 15 minutes, if oleum is used at the rate of 23.6 pounds per minute. Since the cylinders may be arranged in batteries, the screen may be continued for any period of time. The Tank Corps rather favor phosphorus rifle grenades for producing a smoke screen at a distance from the tank.

PURPOSE OF SMOKE SCREEN

Smoke screens may be employed with one or more of the following objects in view:

(1) To mask known enemy observation posts and machine gun nests; to conceal the front and flanks of attacking troops, concentration of guns and tanks, roads and concentration points; to blind the flashes of batteries in action and to hamper aerial observations.

(2) As a feint to draw the enemy’s attention to a front on which no attack is being made, so as to hold his troops to their trenches, or to induce him to expend ammunition needlessly and to put down a barrage in the wrong place.

(3) To simulate gas and force the enemy to wear his mask. Gas should occasionally be mixed with smoke, to impress upon him the belief that it is never safe to remain in a smoke cloud without wearing his mask.

(4) In rolling or mountainous country, to fill valleys with smoke and thereby conceal the advance from all observation, including aerial.

(5) To cover the construction of bridges, trenches, etc., in the face of the enemy.

THE TACTICAL VALUE OF SMOKE

The pall of smoke that hung over every battlefield of the Civil War made a profound impression upon Fries when, as a boy, he first read of those battles. However, practically every reference made to smoke treated it as a nuisance. It obscured the field of vision and interfered with troop movements as well as with the aiming and firing of rifles and cannon, though due to their short range this was not so serious as it would be nowadays. Nevertheless so deeply was this interference appreciated that the most earnest efforts were made to discover a smokeless powder. This, as the world well knows, was developed with great efficiency during the latter part of the nineteenth century. With the development of the smokeless powders came also a better understanding of the action of powder, whereby the velocity of projectiles, and consequently the range and accuracy, were greatly increased. This increased range and accuracy of guns forced a consideration of protection,—and concealment is one form of protection.

The Navy would appear to have been the first branch of the American forces to realize how valuable a smoke screen may be. Thus Fries, in August, 1913, had the interesting experience of witnessing a week’s maneuvers at the eastern entrance to Long Island Sound between the Navy and the Coast Artillery. During that week the Navy carried out extensive experiments with smoke screens both by day and by night. The smoke in all cases was generated by smothering the fires on destroyers or other ships, thus causing dense clouds of black smoke to be given out from the funnels.

After the World War had been in progress some time and particularly about the time the United States entered it, a determined search was begun for more efficient smokes and more efficient smoke producers.

In the Navy, smoke screens were expected to be established by small craft behind which larger vessels could maneuver for position and range. These screens were also established for the purpose of cutting off the view of enemy submarines or other vessels, thus allowing merchant ships or even warships when injured or outclassed to escape.

The Army was much slower to appreciate the value of smoke. In fact, apparently no army really realized the value of a smoke screen until after gas warfare became an accomplished fact. As is well known, the evaporation of the large quantity of liquid used in wave attacks caused a cloud of condensed moisture. This is what gave rise to the designation “cloud attack.”

English regulations for defense against gas in the early days called for every man and animal to stand fast upon the approach of a gas cloud and remain quiet until the cloud had passed. Thus casualties were reduced to a minimum and the English were fresh to receive the attack that was frequently launched immediately after the cloud had passed. The Germans finally thought of the plan of sending over a fake gas attack. In that way they simply produced a smoke cloud that looked like a gas attack. Naturally the English stood fast as before. The Germans attacking in the fake cloud naturally caught the British at a complete disadvantage with consequent disastrous results to the latter.

But that was a game at which two could play. About this time the value of white phosphorus for producing a smoke screen was taken up by the British and large numbers of 4-inch Stokes mortar shells were filled for that purpose. All armies then began to experiment with smoke producing materials. Most of these were liquid. Of them all, as has been stated before, white phosphorus, a solid, proved the best. Toward the close of the war these smoke screens began to be used to a considerable extent for the purposes given above. No one who has engaged in target practice and encountered a fog, or who has hunted ducks and geese in a fog needs to be told of the difficulty of hitting an object he cannot see.

The First Gas Regiment proved its worth and won everlasting glory by using the Stokes’ mortars of the British with their phosphorus bombs for attacking machine gun nests. The white phosphorus in that case had a double effect. It made a perfect smoke screen, thereby making the German machine gun shots simply shots in the dark, while at the same time the burning phosphorus forced the gunners to abandon their guns and surrender. Thus phosphorus played and will play in the future the double rôle of forming a defensive screen and of viciously attacking enemy troops. This phosphorus, which catches fire spontaneously, burns wet or dry, total immersion in water alone sufficing to put it out. This means of extinguishing the flames being almost totally absent on the battlefield, it can be truthfully said that burning phosphorus is unquenchable. The burns are severe and difficult to heal. For these reasons white phosphorus will be used in enormous quantities in any future war.

All armies have begun to realize this value of smoke. In the future it will be the infantryman’s defense against all forms of weapons and it will be used on every field of battle, by every arm of the service and at all times, day or night. It is even more effective in shutting out the light from searchlights, star bombs and similar illuminants for use in night attacks than it is in daylight. With this straight use of smoke for protection will go its use along with poisonous gases. Every smoke cloud will be poisonous or non-poisonous at the will of the one producing the cloud, and this will be true whether it is produced from artillery shell, mortar bombs, hand grenades, smoke candles or other apparatus. Thus smoke and gas together will afford a field for the exercise of ingenuity greater than that of all other forms of warfare. The only limitation to the use of smoke and gas will be the lack of vision of commanders and the ignorance of armies.

Proper recognition and aid given to chemical warfare development and instruction in peace are the only methods of overcoming these limitations. In this, as in all other development work, the most serious obstacle comes from the man who will not see, whether it be from a lack of intelligence, laziness or inbred opposition to all forms of advancement.

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