TOPAZ. 



!5 



Rutile (Ti0 2 ). 



(Flat plate 8 mm. long, tapering from 1.5 to 1.8 mm. wide and 0.25 mm. thick. Energy 



supplied, 6 watts. Curve b, fig. 73. Transmission, Carnegie Publication No. 65, p. 67.) 



This mineral was heated to a bright red color corresponding to a 

 temperature of perhaps 1000 . The emission spectrum shows maxima at 

 2.4, 3.2, 5,5, and 7.0//. The transmission spectrum is too low to show 

 these as absorption bands; but the band at 3.1 / is visible in the trans- 

 mission spectrum of brookite (Ti0 2 ). This substance is a good conductor 

 of electricity at this temperature, but a very poor radiator of light rays. 



3 4 5 6 



Fig. 73. Beryl (a); Rutile. 



'M 



Topaz [(AlF) 2 Si0 4 ]. 

 (Rod 13 mm. long, 2 to 2.5 mm. diameter. Energy supplied, 0.05 and 0.07 ampere, 

 probably 15 to 21 watts. Curves a and b, fig. 74.) 



This rod was built up from the massive transparent mineral, and 

 rendered conducting by the vapor set free from the albite cement used to 

 secure the platinum terminals to the rod. The vapor left a narrow con- 



