Reacciones Sna Organica

SMOKE-FORMING CHEMICALS
KIRBY E. JACKSON* Department of Chemistry, Georgia School of Technology, Atlanta, Georgia Received November 4, 19S7

According to Prentiss (261) the colloidal state of matter is characterized by an intimate admixture of at least two phases,—the dispersed phase and the dispersion medium. By a dispersion of this kind is meant the regular distribution of one substance inanother in such a way that the individual particles of the one substance are suspended separately from each other in the second substance. In this sense smoke is to be regarded as a twophase colloid whose dispersion medium (the air) is in the gaseous state and whose dispersed phase is a solid or liquid. So-called colloidal solutions of this kind have physical and chemical behavior entirelydifferent from that of normal solutions, in that the size of the particles may vary within certain limits without causing the solution to lose its colloidal character. The particles of a smoke or fog vary in size from those just large enough to be perceived by the unaided eye to those that approach the size of a single molecule. In general, smoke particles are intermediate in size between dust particles(ICT4 cm.) and gas particles (1O-7 cm.) and average about 1O-5 cm. in diameter. As a rule, the smaller the particles in a given quantity of smoke, the greater is their obscuring power; hence the aim is to generate a smoke consisting of the maximum number of particles of medium size. Since smoke is a suspension of minute solid or liquid particles, it is not a true gas and does not follow the law ofgaseous diffusion. However, owing to the collisions of the molecules of air with the smoke particles, the latter exhibit Brownian movements, as a result of which they gradually diffuse and spread. Because of their greater mass and inertia and the resistance of the air, the larger particles of smoke diffuse more slowly than the smaller ones. But, compared with the effects of wind and convectioncurrents, diffusion plays an almost negligible part in the dispersion of smokes; even in a very dense smoke the weight of the smoke particles is only a small fraction of 1 per cent of the weight of the air it occupies, so that a smoke cloud is distinguished from the surrounding atmosphere only by the small amount of suspended foreign material.
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KIBBY E. JACKSON

If smoke is released in warm air, it will rise as the warm air expands. If released in cold air, where these upward convection currents are absent, the smoke will spread out in a horizontal layer and cling to the ground. The movement of the cloud is therefore merely the movement of the air, which accounts for the characteristic behavior of smoke clouds.Since floating smoke particles are themselves heavier than air, they gradually fall, although at a very slow rate which varies with the size of the particles. Thus, according to Grey and Patterson (120), a smoke particle having a diameter of 1O-4 cm. falls about 0.071 in. per minute, which is so slow as to be negligible for practical purposes. In the same manner as particles in colloidal solution,smoke particles tend to unite and increase in size by cohesion and coalescence as they come in contact with each other by Brownian movements or air currents. This agglomeration takes place much more rapidly in a dense than in a thin smoke. When the smoke particles are completely dry, agglomeration is not observed, but when the particles are liquid, or of a deliquescent solid with condensed surfacemoisture, this is more pronounced. The increase in the number and size of the larger at the expense of the smaller particles increases the rate of settling and decreases the concentration of the cloud. Also, the smaller smoke particles vaporize more rapidly because their surfaces are greater in proportion to their weight. Thus it follows that a smoke is most stable when the particles are of the...