The reproducibility and rate of coagulation stearic acid smokes

In a previous communication the validity of the equation σ = σ0 + Kt as a general expression for the coagulation of smokes was discussed. In this equation a is the particulate volume of the smoke at time t, and σ0 that at zero time. It was shown, however, that the value of the constant K varied with the weight of material dispersed, the rate of coagulation, and hence K, being greater, the smaller the weight concentration. This was found to be in general accordance with the theory of Smoluchowski when applied to smokes. At that time, however, a smoke of a given weight concentration could not be accurately reproduced, and consequently it was not possible to estimate how far the agreement with theory was qualitative rather than quantitative. In the same paper preliminary experiments were described which indicated that by blowing a stream of air over the material during dispersal, not only was it easier to reproduce a smoke of a given weight concentration, but also the uni­formity of size of the particles composing the smoke was increased considerably. This method of producing smokes has now been worked out in detail and it has been found possible to obtain data enabling a quantitative comparison with theory to be made.Method . The material was dispersed from a boat, a drawing of which is shown in fig. 1. A is a basket made of fine silver gauze and screwed on to a narrow brass rim B, which is again screwed on to a keel-shaped piece of brass C. C is so shaped that it rests during dispersal on two parallel quartz tubes ⅜ inch in diameter, containing nickel chromium wire which can be heated electrically. At the end of the boat is a hole D into which a thermocouple can be inserted in order to obtain some idea of the temperature of dispersal. The boat and quartz tubes are contained in a brass box E (fig. 2) which is clamped and ground on to a brass tube F. The other end of F is wound with nickel chromium wire G which is heated during dispersal and assists in the production of a uniform size of particle. The action is probably to be attributed to a partial evaporation of the particles, followed, as the smoke cools, by a recondensation. This latter will take place much more rapidly on the smaller particles and should lead to an increase of uniformity. The air stream, which is carefully filtered to remove dust, passes through an anemometer and thence enters the heater at H. The volume or velocity of the air passing over the boat can thus be estimated.