6 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



physics as established under ordinary conditions retain their validity 

 at temperatures exceeding iooo C, while of the chemical behavior 

 of substances at these temperatures chemists can tell us little more 

 than that affinities are radically different from those observed at ioo. 

 Similarly elasticians can discuss the small strains in a building or a 

 bridge with some approach to completeness, but they do not even 

 make the attempt to deal with deformations which are sensible to 

 the eye and which are almost universal in geological exposures. 



It was in recognition of the need for researches in physics which would 

 throw light on geological problems that Dr. Carl Barus was ap- 

 pointed physicist on my staff in the United States Geological Survey 

 as far back as 1880, and that a physical laboratory was estab- 

 lished under that Survey in 1882. This was discontinued in 1S92, 

 not because its importance was underestimated by the Director, 

 but on account of a failure of appropriations. The laboratory was 

 reestablished in 1901 because it was felt that without the aid to be 

 derived from physical determinations the efficiency of the Survey 

 must suffer. There was nothing novel in the appreciation thus dis- 

 played of the importance of physics to geology; indeed, several great 

 geophysical problems have been recognized by natural philosophers 

 for more than a century ; and their difficulty, not their unimportance, 

 has stood in the way of experimental investigation. 



The field geologist meets with phenomena in all the ruggedness of 

 their utmost complexity, and he is sometimes tempted to face and 

 make an assault upon the situation as he finds it. A little consider- 

 ation shows that in such circumstances a frontal attack must lead to 

 disaster. The outposts must be overcome one by one. We must 

 patiently begin with the simplest problems that can be devised and, 

 aided by the most perfect appliances known, study them exhaustively 

 before proceeding to more difficult and complex cases. 



In a plan submitted to the Director when the new physical labora- 

 tory of the Survey was first contemplated, I laid especial stress upon 

 the study of isomorphism and eutexia. These subjects, with the 

 determinations of thermal constants which they imply, have occu- 

 pied the attention of the physical laboratory during the greater part 

 of the time since its reestablishment, and will continue to take the first 

 place in the researches there undertaken. 



It would appear that the relations between liquids must be reduci- 

 ble to very general groups. Liquids must either be miscible or im- 

 miscible, and miscible liquids must exhibit either isomorphic proper- 

 ties or eutectic ones. It is possible that magmas are in some cases 



