12 ABSORPTION SPECTRA OF SOLUTIONS. 



compound between the salt and the hydrochloric acid liberated as the 

 result of hydrolysis. 



Wyrouboff : and Le Chatelier ~ show that Engel's view is untenable. 



W. N. Hartley, 3 in his elaborate investigations on absorption spectra, 

 has included a number of salts of cobalt. His experimental work con- 

 sisted in observing and photographing the spectra of a large number of 

 solutions of chlorides, bromides, iodides, nitrates, etc., of a fairly large 

 number of metals, including cobalt. Some of the more interesting and 

 important conclusions at which he arrived are the following, stated nearly 

 in his own words. 



When a definite crystalline hydrate is dissolved in a non-aqueous sol- 

 vent, upon which it does not act chemically, the molecules of the salt re- 

 main unchanged in chemical composition. 



In a series of anhydrous salts which do not form definite crystalline 

 hydrates, the effect of rise in temperature up to 100 C. does not produce 

 any alteration in their absorption spectra, other than that which results 

 with substances which undergo no chemical change with such rise in 

 temperature. The change in question is usually an increase in the inten- 

 sity of the absorption, or a slight widening of the absorption bands. 



Crystallized hydrated salts dissolved in a minimum amount of water at 

 20 C. undergo dissociation by rise in temperature. The extent of the dis- 

 sociation may proceed as far as complete dehydration of the compounds, 

 so that more or lees of the anhydrous salt may be formed in the solution. 



The most stable compound that can exist in a saturated solution at 

 16 C. or 20 C. is not always of the same composition as the molecule of 

 the crystallized solid at the same temperature, since the solid may undergo 

 a partial dissociation from its water of crystallization when the molecule 

 enters into solution. When a saturated solution of a colored salt under- 

 goes a great change in color or any remarkable change in its absorption 

 spectrum upon dilution, the dilution is always accompanied by marked 

 heat evolution. 



Hartley 4 at the close of his paper on "The Absorption Spectra of 

 Metallic Nitrates" has the following significant paragraph. 



The ultimate conclusion drawn from this work is that the operations of dissolving a 

 salt and diluting the solution do not cause a separation of the compound into ions, but 

 only a dissociation of such a character that the molecule is shown to consist of two parts, 

 the movements of the one being influenced by those of the other, so that the molecule of 

 the salt is, in fact, not completely resolved into ions, but is in a condition of molecular 

 tension. The application of external energy, such as light or electricity, may, however, 

 readily cause a separation such as may be brought about by electrolysis or by static elec- 

 tricity, and in some instances, by photographic action. 



Ostwald 5 thinks that the red color of solutions of salts of cobalt is due 

 to the cobalt ions. 



Bull. Soc. Chim. (3) 6, 3 (1891). 

 2 Ibid. (3)_, 6, 84 (1891). 



3 



Dublin Trans. (2), 7, 253-312 (1900), and Journ. Chem. Soc., 81, 571 (1902); 83, 



221 (1903). 



4 Journ. Chem. Soc., 83, 245 (1903). 

 6 Grundlinien d. anorg. Chem., 620. 



