556 THE POPULAR SCIENCE MONTHLY 



has completely revolutionized our knowledge of the double salts and of 

 geological formations. One of the most beautiful applications of the 

 phase rule is found in van't Hoff's investigations of the oceanic salt 

 deposits at Stassfurt/^ in which from a laboratory study of the equi- 

 librium obtaining between the sulphates and chlorides of sodium, cal- 

 cium, magnesium and potassium the great chemist was able to recon- 

 struct the past history of the formation of the earth's crust from the 

 primeval ocean, giving even the limits of time, the pressure and the 

 probable temperature at which the water evaporated. The importance 

 of such methods in mineralogy and geology is self-evident and clearly 

 as extensive as the subjects themselves. In metallurgy the work of 

 Eoozeboom, Le Chatellier, Sorby, Stead and others on steels, bronzes, 

 tins, alloys and ingots of metal generally have, with the aid of the 

 microscope, given us most valuable knowledge of the continuous chem- 

 ical changes and " diseases of metals " going on in these substances, 

 which, without the guidance of the phase rule, were formerly in- 

 vestigated in an aimless and haphazard way, at enormous expense and 

 waste of time. Investigations of the very complicated equilibria in 

 such " solid solutions "''^ as carbon-steels, nickel-steels, cobalt-steels, 

 etc., have explained the causes of brittleness and crystalline structure 

 in steel-rails through extended use, and how such rails can be renewed 

 by prolonged heating at high temperatures. " The variations of the 

 engineering properties, such as tensile strength, torsional resistance, 

 ductility, etc., with varying concentration and varying heat treatment, 

 is a subject which can only be worked out satisfactorily with the phase 

 rule as a guide," says Bancroft, and he adds, " we do not yet know one 

 half the properties of our structural metals." The establishment of the 

 true constitution of Portland cement is another telling application of 

 the phase rule and it is thought that it will give us " clearer ideas as to 

 the strength of cements and the elasticity of clays." Lord Kelvin 

 expressed the hope that some day the architect might be in effect a 

 sanitary engineer,^" and Bancroft predicts that " the time will come in 

 our engineering schools when the subject known as ' materials of 

 engineering ' will have to be taught by the chemist rather than by the 

 engineer."^^ 



In agricultural chemistry the phase rule serves as a guide in the 



" Van't Hoff, " Lectures on Physical Chemistry," Chicago, 1907. 



" Solid solutions are solids dissolved in solids, and were first described by 

 Tan't Hoflf {Ztschr. f. phys. Chem., 1890, V., 322), who found that when certain 

 solutions are frozen, the separated solid is not the pure solvent, but a mixture 

 of the selvent and the solute, i. e., a solid solution, of which we have examples 

 in the alums, glasses, colored and hyaline minerals, alloys and the " ice flowers " 

 of the Antarctic regions. 



«" Kelvin, "Popular Lectures," 1894, II., 210. 



"^ Bancroft, J. Phys. Chem., 1905, IX., 209. 



