DISCUSSION OF EVIDENCE. 183 



the general principle that rise in temperature breaks down aggregates 

 formed with evolution of heat, and most aggregates are formed with 

 heat evolution. 



If the original temperature is restored, the original conditions of 

 equilibrium are reestablished and the initial solvates reformed. If 

 there were only a few definite hydrates in any given aqueous solution, 

 each of these would probably be stable over a definite range in tem- 

 perature, and the changes in their composition would probably take 

 place by jumps. This would produce correspondingly irregular changes 

 in the absorption spectra, and not the regular transitions which were 

 noted. 



SPECTROPHOTOGRAPHY OF CHEMICAL REACTIONS. 



The effect of adding an acid to uranium salts of another acid was 

 studied at some length. Thus, uranyl nitrate was treated with sul- 

 phuric, hydrochloric, and acetic acids; uranous and uranyl acetates 

 with various acids ; a number of uranous salts and neodymium acetate 

 with nitric acid, and so on. The salts and acids were selected so as 

 to show the greatest spectroscopic changes. The action of nitric acid 

 on uranous salts is especially interesting. 



The spectrophotographs of chemical reactions show that, as the 

 salt of one acid is transformed into the salt of another acid, the changes 

 produced in the spectra are gradual. For example, when uranyl 

 nitrate is transformed into uranyl sulphate, the uranyl nitrate bands 

 gradually shift into the sulphate position. The details and data 

 bearing on this point are given in Publication No. 130 of the Carnegie 

 Institution of Washington. The addition of a large amount of sul- 

 phuric acid to a small amount of a solution of uranyl nitrate in nitric 

 acid, showed admirably the gradual shift of the bands from the nitrate 

 to the sulphate position. 



The addition of a small amount of nitric acid to uranous acetate, 

 does not appreciably oxidize the uranous salt. The uranous bands 

 are shifted towards the violet. 



The gradual shift of the absorption bands as one salt of a metal is 

 transformed into another salt by the addition of more and more free 

 acid is very important. 



The work done in my laboratory, which, up to the time we are now 

 discussing had had to do with about 5,000 solutions, had shown that 

 any given series of absorption bands corresponds to a definite chemical 

 condition of the dissolved substance. When a salt is treated with 

 acid, the absorption bands of some of the salts shift gradually over to 

 the position occupied by the bands corresponding to the new salt of 

 the metal with the acid in question. In such a case the absorption 

 bands can be made to occupy any position between the initial and 

 final positions. It therefore seems probable that, when a salt of one 

 acid is transformed in this way into a salt of another acid, a series of 



