120 
Notes . 
heating. To a given temperature there corresponds a constant 
coefficient of diffusion of carbon in iron. This law is only true 
when the iron is converted into steel. When cast-iron begins to 
be formed — that is a little before the iron becomes brittle — the 
absorption of carbon decreases. Silica ranks among the bodies 
most easily diffusible in carbon. By heating platinum in lamp- 
black containing 60 per cent of precipitated silica, we obtain a 
crystalline body, Si 2 Pt 3 , of the specific gravity 14*1, and melting 
at about the same temperature as common glass. 
MM. Berthelot and Vieille, in a paper on the Speed of Pro- 
pagation of Explosive Phenomena in Gases (“ Comptes Rendus ”), 
observe that the general mean speed of propagation in a tube, 
whether curved or straight, 0*005 metre in diameter, is 2810 metres 
per second. The material of the tube is indifferent. The speed 
is sensibly the same whether the tube is open or closed. The 
differences observed in tubes of different lengths fall within the 
limits of error. Under the circumstances of the author’s expe- 
riments the speed of propagation of detonation — either with a 
mixture of oxygen and hydrogen, or of oxygen and carbon oxide 
—is independent of the degree of pressure. The introduction 
of an inert gas retards the detonation. 
