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particles. Chart XVI shows the effect of humidity on the T.O.P. of a

uroint or womtr per tti.Ffc of" Air Chart XVI.—B. M. standard smokr mixture (humidity curve).

A satisfactory smoke cloud requires, in the first place, density; a relatively thin layer of the smoke must completely obscure any object behind it. In the second place, the cloud must be inherently stable; it must not quickly dissipate or dilute, nor must it settle out. Third, the 1.500

T5 80 85 90 95 100

Degree» Fahrenheit

Chart XVII.—B. M. standard «moke mfxtuiv mfwraturi* nirve).

cloud must easily be produced without complicated apparatus or difficulties of manipulation. Finally, the materials required for producing the cloud must be readily available, easy of transportation, and not dangerous to handle.

Any smoke fulfilling these requirements must be composed of extremely small liquid or solid particles dispersed in the air. The individual particles must be large enough to disperse and diffuse light; but otherwise the smaller the particles, the greater the obscuring power for a given concentration of smoke in air and the more stable the smoke. The diffusing power of a given particle is probably not greatly influenced by its size, provided the particle is large enough to diffuse light at all. The problem of smoke formation, therefore, reduces itself to the problem of producing a suspension of extremely small particles of the smoke-forming substance in air. Successful smoke production depends upon appreciation und application of the following generalization.

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