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LII. Selective Absorption. By William Ackroyd*. 



AT present much attention is being paid to absorption- 

 spectra for technological as well as purely scientific 

 reasons ; but in the absence of classification and of a general 

 theory accounting for the facts observed, the whole subject is 

 surrounded by a haze which it were well for progress to dispel. 

 Much perplexity and not a few mistakes have arisen from the 

 confusing of two kinds of absorption, or at least two different 

 manifestations of the same phenomenon, which hitherto have 

 not been sufficiently contradistinguished. These we propose 

 to classify on the basis of the following typical experiments; 



1. Light is transmitted through a thin layer of poiassic di- 

 chromate at the normal temperature, and again at a little below 

 its fusing-point. We obtain spectra approximately represented 

 by Nos. 1 & 2. It will presently be shown that this increase 



of absorption at the higher temperature is the result of struc- 

 tural alterations in the dichromate. We shall therefore call 

 this kind of absorption structural absorption. 



2. A glass cell, 15 centims. x 3 centims., contains cupric 

 sulphate in solution. Light sent through the 3-centims. direc- 

 tion gives spectrum No. 3, whilst light transmitted through 

 the 15-centims. direction gives spectrum No 4. We have here 



* Communicated by the Physical Society. 



