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.