SECT. ii. ABSORPTION OF INVISIBLE RAYS. 65 



magnesium gave the shortest spectrum, ending in a very 

 bright line, beyond which however excessively faint 

 light extended to a distance equal to that of the long 

 spectra. Aluminium on the other hand exceeded all 

 the other metals in richness of the rays of the very highest 

 refrangibility. All the strong lines mentioned lie in 

 that part of the spectra. 



In the course of these experiments Professor Stokes 

 observed that even quartz of a certain thickness is not 

 transparent to invisible lines of the highest refrangi- 

 bility, for the* highest aluminium line, which is double, 

 could only be seen by rays passing through the edge of 

 the prism. This leads to another branch of the subject, 

 namely, the absorption of the invisible rays by solids, 

 liquids and gases. Mr. Wm. Allen Miller has shown from 

 his own experiments that bodies pervious to the chemi- 

 cal rays in the solid form, are so also in the liquid and 

 gaseous form ; that colourless transparent solids which 

 absorb the photographic rays, absorb them more or less 

 also in their liquid and gaseous states. He has more- 

 over found that the following substances have the same 

 maximum transparency : rock crystal, ice, and fluor 

 spar among solids, water among liquids, the three ele- 

 mentary gases and carbonic acid among gaseous sub- 

 stances. The most opaque to the invisible rays are, 

 nitrate of potash, bisulphide of carbon, and sulphuretted 

 hydrogen. It appears that a thin plate of mica is 

 intensely opaque to all the invisible rays except a small 

 portion of them of the lowest refrangibility. 



The absorptive property however is partial : an ab- 

 sorptive substance either cuts off a portion of the light 

 of a fluorescent spectrum or stripes it with dark lines : 

 each substance absorbs rays peculiar to itself. Those 

 employed by Professor Stokes were the alkaloids and 

 glucosides, and he assumed the spectrum of tin for their 

 examination because it has a long interval of continuity. 



VOL. I. P 



