SPECTROSCOPIC EVIDENCE FOR THE HYDRATE THEORY. 237 



a constant amount of copper bromide. A careful study of the negatives 

 corresponding to the mixed solutions of the kind just described shows that 

 the absorption bands of these solutions conform perfectly to the theory in 

 question and do not present any exceptions. Earlier investigations have 

 shown that aluminium chloride is a much stronger dehydrating agent than 

 calcium chloride. This being the case, we should expect to find that a given 

 widening of an absorption band of a colored salt, so called, which had been 

 caused by the addition of a certain amount of calcium chloride, would be 

 produced by a smaller amount of aluminium chloride. 



Consider the case of cobalt chloride with the other two chlorides just 

 mentioned. [See plates 9 (a), 11 (6), and especially plate 12.] The pairs of 

 solutions were made up so as to have the same colors when viewed in their 

 bottles. The successive widths of the absorption band in the green, for 

 the solutions which contained calcium chloride, are slightly, but only 

 slightly wider than those of the band of the corresponding solutions of the 

 aluminium chloride series. On the contrary, the concentrations of the cal- 

 cium chloride were much greater than the concentrations of the aluminium 

 TABLE 11" chloride, everything else being kept constant. 



The concentrations are compared in table 117. 

 The region of absorption in the ultra-violet is 



not relevant, because of the intense absorption of 



CaCl. 



1.676 

 2.091 

 2.515 

 2.617 

 2.830 

 3.143 

 3 .555 



A1CL, 



1.118 



1 394 the snor t waves by aluminium chloride alone. 



l .676 Another illustration is afforded by the solutions 



1 781 



1 .887 f copper chloride, together with the chlorides of 



2.100 calcium and aluminium. (See plates 13 and 17.) 

 9 370 



A careful comparison of the negatives shows that 



the absorption produced by the most concen- 

 trated solution of aluminium chloride certainly extended farther into the 

 visible spectrum than the corresponding band for the solution of calcium 

 chloride, which was the third from the end of its series. The concentrations 

 of the calcium and aluminium salts were, respectively, 3. 518 and 2.482. The 

 same fact may be brought out very sharply as follows : The two concentra- 

 tions for the calcium chloride series, which come nearest to the concentration 

 of the strongest aluminium chloride solution, and between which the latter 

 concentration falls, are 2.435 and 2.706. The photographic strips pertaining 

 to the calcium chloride solutions are the 10th and llth of plate 13, counting 

 from the numbered scale towards the comparison spectrum. A single glance 

 shows that both of these bands of transmission extend to much shorter wave- 

 lengths than the last strip of plate 17. 



That it is in general much more difficult to remove the last molecules of 

 water from a given compound than the first is a well-known fact. Illustra- 

 tions of this phenomenon are so numerous as to make it almost superfluous 



