merwin: media of high refraction 37 



iodide slowly dissolves precipitated arsenic trisulphide, forming 

 an orange-colored liquid. After some days crystals separate, but 

 while fresh — or after the heating of an old preparation which is 

 not discolored — the solution can be used for accurately matching 

 the refractive index of a substance, and then standardized by 

 means of a goniometer^ or a spectrometer and prism. 



Resin-like substances, n =1.68 to 2.10. Piperine, one of the 

 least expensive of the alkaloids, can be obtained in very pure, 

 straw-colored crystals. When melted it dissolves the tri-iodides 

 of arsenie and antimony very freely. The solutions are fluid at 

 slightly above 100°, and when cold are resin-like. The red color 

 of the iodides is present in the solution. The color, however, is 

 of such a character that determinations of refractive index made 

 in the solutions in white light are almost as accurate as those 

 made in sodium light. If less than 7 per cent of the iodides is 

 present this solution should be examined thru a film of a 7 per 

 cent solution. A solution containing 3 parts of antimony iodide 

 to 1 part of arsenic iodide with varying proportions of piperine 

 is easier to manipulate than one containing either iodide alone. 

 The diagram, figure 1, gives the necessary data concerning com- 

 position and refractive index for sodium light. In preparing, the 

 constituents, in powder of about 1 mm. grain, should be weighed 

 out, and then fused over — not in — a low flame. Three-inch test 

 tubes are suitable. Stirring with a glass rod is essential. A 

 little of this material is placed on a glass slip with some of the 

 powder to be examined, it is then warmed and pressed thin under 

 a cover-glass. For use with the more highly refracting mate- 

 rials the powder must be very fine to permit a very thin film to 

 be made. An artificial light is advantageous for observing the 

 deeply colored films. The iodides should be examined under the 

 microscope for mechanical impurities. 



Mixtures of amorphous sulfur and arsenic trisulflde, n =2.1 to 

 2.6. These mixtures are much lighter colored than corresponding 

 ones of sulfur and selenium, but they are less easily standardized 

 and manipulated. They should be used only in cases requiring 

 greater transparency than the sulfur-selenium mixtures. They 



1 See Am. Jl. Sci. 34: 46. 1912. 



