56 



INFRA-RED TRANSMISSION SPECTRA. 



red; it was taken from a Le Chatelier pyrometer and is described as a 

 copper oxide glass. Although thinner, the second sample is the more 

 opaque in the region of i to 2.5 ,u. The transmissivity of the monochro- 

 matic red glass is very unusual. In the region of 1 to 2.5 p. the metal 

 which is used to color the glass renders it more permeable to heat waves 

 than is ordinary, visually transparent glass. The band at 2.9 p appears 

 to be intensified by the presence of the metal, which behaves like the 



100% 



90% 



80 



70 



60 



c 

 



' <f> 

 (0 50 



'I 



(0 



1- 

 t- 



30 



20 



10 



*M 



Fig. 39. Green glass. 



12 3 4- 



Fig. 40. Ruby glass (a) and (6); Sphalerite. 



colloidal suspensions just mentioned. In fact, it is a pertinent question 

 whether the red color in glasses is due to the presence of metals, such as 

 copper and gold, in the colloidal condition. The monochromatic red 

 glass was found to have a uniform reflecting power (about 4 to 5 per cent) 

 throughout the spectrum to 8 [x. This seems to show that the transparent 

 region at 1 to 2 ti is not due to the same cause that produces an abnormal 

 transparency on the short wave-length side of a region of anomalous dis- 

 persion. 



Black Glass. 



(Fig. 41. Curve a, = 2.48 mm.; curve b, Schott's Rauchglass No. 444, III, ts-6 mm.) 



Curve a gives the transmission of a dark "neutral" glass, colored with 

 the oxides of cobalt and nickel. This glass has a uniform absorption 

 throughout the visible spectrum. There are small absorption bands at 

 2 and 3.5 p., respectively. Curve b gives the transmission of a very dark 

 glass, which shows an absorption band in the region of 1.5 ,. 



Considered as a whole, these glasses can be divided into two groups, 



