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.