AS STUDIED BY MEANS OF THE RADIOMICROMETER. 



39 



of neodymium chloride; it contains, for a given thickness, only about 90 per 

 cent as much water as the same thickness of pure water. It is, then, obvious 

 that in the longer wave-lengths a correction term must be introduced for this 

 difference. This was practically negligible with salts of neodymium, since 

 these do not seem to have any bands in the region where water has appre- 

 ciable absorption. 



Salts of praseodymium have bands in the infra-red, at least as far as 2ju. 

 In such cases the above correction must be introduced. This correction 

 can be introduced in either of two ways. We can take the specific gravity 

 of the solution and from the concentration calculate the amount of water 

 present. We can then use a "water" vessel of suitable thickness. For 

 example, if the very concentrated solution in question contains only 90 per 

 cent of water, and we use a vessel for the solution which is 10 mm. thick, we 

 must use a vessel for the water which is only 9 mm. thick. In this way the 

 beam of light is made to pass through the same amount of water both in the 

 case of the solution and of the solvent, and the absorption due to water is, 

 therefore, the same in the two cases. 



The second method of procedure is to allow the "water" vessel and the 

 "solution" vessel to be of the same thickness, and to apply mathematically 

 the proper correction to the results obtained. 



OAji 



0.6//. Q.I/jl 



Fig. 1. 



i.cy 



