AND THE ABSORPTION SPECTRUM OF GOLD. 51 



LANGLEY'S bolometer with photographic auto-registration of the results. These 

 results were in general as follows : 



The specimen of ruby-red gold glass almost totally absorbs the light of the short- 

 wave-length side of D, rapidly increases to the full transparency of the ordinary 

 colourless (unflashed) glass at about A, and continues as transparent as this ordinary 

 glass to about 2' 5 p. The saphirine or blue glass coloured by gold, cuts off the light 

 to near C, then rises very rapidly to great transparency at and beyond A. 



The red colloidal gold liquid No. 1, 20 milligs. metallic gold per litre, contained in 

 a cell with end plates of thin microscope glass, 4 "5 centims. apart, produces great 

 general absorption in the visible spectrum, though not reaching to the point of 

 completely, or almost completely, extinguishing any rays included within the region 

 of spectrum studied, as was the case with the glass specimens. The absorption of 

 this liquid becomes practically identical with that of distilled water at and beyond A. 



The violet-blue colloidal gold liquid No. 2, 50 milligs. gold per litre, and the 

 greenish-blue liquid No. 3, 75 milligs. gold per litre, behave on the whole like liquid 

 No. 1, except that they diminish the radiations throughout the spectrum to a very 

 great extent, as if by the interposition of opaque obstacles to the rays. Liquid No. 3 

 appears relatively less transparent in the visible spectrum, besides being generally 

 less transparent throughout the spectrum. 



I regret that the blue-print tracings of the bolometer curves are so faint as not to 

 allow of photographic reproduction on a reduced scale. 



For the microscopic and spectroscopic photographs I have to thank the kind 

 assistance of Professor A. H. TUTTLE and Dr. W. J. HUMPHREYS of this University. 



H 2 



