4f> M> noin and Larsen — Mixtures of Amorphous 



The mixtures are prepared by weighing into a 3-inch test- 

 tube the required amount of Se, heating till tlie Se is thor- 

 oughly fused, allowing to cool, adding the S and heating over 

 a low flame just sufficiently hot to allow thorough mixing with 

 a glass rod. As the material cools it may be gathered on the 

 rod, removed, and cut into small fragments, which may be 

 kept permanently in the stoppered tube. A gram or two is 

 sufficient for examining a hundred preparations. 



For use with the microscope a small piece of the mixture 

 and a little of the mineral powder are heated on a glass slip 

 under a cover-glass over a small flame till the mixture is fluid. 

 The powder and liquid are then mixed and pressed into a thin 

 film. The film is then heated during 15 to 30 seconds till 

 bubbles begin to appeal-, again pressed thin and cooled. If 

 care is taken no appreciable amount of sulphur will be vapor- 

 ized from the film. 



In the preparation of the mixtures, artificially crystallized 

 sulphur should be used. Flowers of sulphur is satisfactory if 

 it contains no mechanical impurities. The powdered or fused 

 selenium furnished for this investigation by three dealers has 

 been found to be sufficiently (better than 99 - 7 per cent) pure. 

 It was tested by determining the refractive index of the glass 

 made from the samples, and of that made from different frac- 

 tions of the distilled selenium, and from precipitated selenium. 

 The extreme values obtained in lithium light were 2*712 and 

 2*718. Mechanical impurities were sought for under the 

 microscope in thin films of the glass. They were removed 

 from one sample by distillation in a bent test-tube. In case it 

 is not convenient to determine the refractive index of the 

 selenium directly, its purity may be tested under the micro- 

 scope. A mixture of 73 per cent Se, 27 per cent S, has the 

 refractive index of pure sphalerite for light of wave-length 

 635yu./t. This is the equivalent wave-length of light trans- 

 mitted by a thin film of this mixture. The test is made then 

 by embedding the powder of colorless or amber-colored sphal- 

 erite* in the mixture and determining the relative refractive 

 indices through a screen of the same mixture. If the slide 

 has a temperature of 30°-35° C, or if it is studied through a 

 screen of 85 per cent Se, the sphalerite should have the higher 

 refractive index. 



The refractive index of a mixture, correct to ±*001, may be 

 found on the goniometer by cautiously melting the mixture in 

 the angle between two narrow strips of glass joined firmly at 



* One per cent Cd or J^ per cent Fe raises the refractive index about 001. 

 The presence of 1 per cent or more of Fe produces a deeper color. The 

 presence of disturbing amounts of Cd is very rare and can be known by 

 analvsis. 



