=Titanium Tetrachloride.= Titanium tetrachloride, TiCl₄, is made from rutile, TiO₂, by mixing with 30 per cent carbon and heating in an electric furnace. A carbonitride is formed, which is said to have the composition Ti₅C₄N₄, but the actual composition may vary from this to the carbide TiC. This product is heated to 600-650° C., and chlorine passed through, giving the tetrachloride. It is a colorless, highly refractive liquid, which boils at about 136° C., is stable in dry air and fumes in moist air. The best smoke is produced by using 5 parts of water to one of the tetrachloride, instead of the theoretical 4 parts [which would form Ti(OH)₄.] Since it is more expensive to manufacture and not as effective as silicon or tin tetrachloride, it is used only as an emergency material. =Berger Mixture.= One of the most important smoke materials was the zinc-containing mixture, which was used in the smoke box, the smoke candle, certain of the smoke grenades and in various forms of colored smokes. The basis of this was the _Berger Mixture_, which had the composition: Zinc 25 Carbon tetrachloride 50 Zinc oxide 20 Kieselguhr 5 This formula produced a light gray carbon smoke, with much carbon in the residue. In this mixture the zinc and carbon tetrachloride react to form zinc chloride and carbon; the kieselguhr keeps the mixture solid by absorbing the tetrachloride, while the zinc oxide is practically useless, as its absorbing power is small. In order to accelerate the reaction and to oxidize the carbon, thereby changing the color of the smoke from gray to white, an oxidizing agent was added. Sodium chlorate was chosen for economic reasons. The reaction now proved to be too violent, and the zinc oxide was replaced by ammonium chloride. This cooled the smoke, retarded the rate of burning and added to the density of the smoke, since the obscuring power of the ammonium chloride is high. The kieselguhr was replaced by precipitated magnesium carbonate, which is as good an absorbent, gives a much smoother burning mixture, and also adds somewhat to the density of the smoke by virtue of the magnesium mechanically expelled. The mixture then had the composition: Zinc 34.6 Carbon tetrachloride 40.8 Sodium chlorate 9.3 Ammonium chloride 7.0 Magnesium carbonate 8.3 SIZE OF SMOKE PARTICLES In the problem of smoke production, the size of the particle is of great importance. Being a physical quantity it can easily be correlated with such physical properties as settling, diffusion, coagulation, and evaporation. These factors are more important in connection with toxic smokes, since there the penetration factor must be considered.