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.
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.