Cadmium, and Mercury. 391 



directly reflected and a small amount of red, orange, yellow 

 and green are reflected after passing through the surface layer 

 of crystals. The combined effect of all the reflected light is a 

 lustrous, dark, yellow to yellowish-green color, or dark citrene. 

 Similarly, plane-faced bright crystals, but having diameters 

 of -01'"'" or less reflect about the same amount of blue light 

 directly, but they reflect much more of that which has pene- 

 trated the surface. The resulting color is a brilliantly pure 

 yellow. Massed crystals of the size of the last but with dull 

 faces have a light yellow-brown or citrene color. A powder 

 consisting of crystalline grains of which the surfaces are mostly 

 bright but not plane, such as is formed by grinding, is in- 

 variably a brilliant orange color. In this case there is less 

 direct reflection, and much of the light finally reflected from 

 the interior has penetrated deeper and thus lost more green 

 and yellow than in a powder having plane-faced fragments. 

 Combinations of the physical conditions described cause varia- 

 tions between the extreme colors enumerated. 



The colors of dry, amorphous cadmium sulphide may be 

 explained in much the same way. The amorphous sulphide, 

 however, absorbs more strongly in the yellow and green than 

 does the crystalline. By transmitted light its color is orange- 

 yellow in films -01"" 11 thick, and yellow in films -001 mm thick. 



The globules of which the pulverulent amorphous sulphide 

 consists may have bright surfaces or surfaces dulled by 

 wrinkles. In the former case the colors are most brilliant and 

 pure. Powders consisting of separate globules -0001 to *001 mm 

 in diameter are bright yellow with a tinge of orange ; powders 

 having globules "004 to "007 mm in diameter, or compact aggre- 

 gates of smaller globules, are bright orange-colored. (See 

 Table XII.) 



The lumpy aggregates of dried flocculent precipitates have a 

 duller orange color, owing to a less complete reflection of the 

 light which enters the powder. 



The characteristics of amorphous precipitates formed in 

 various ways are given in Table XII, and the dispersion curve* 

 for the purest material at hand is shown in fig. 8. 



A precipitate made by treating a 10 per cent solution of 

 CdS0 4 4- 20$ H 2 SO t with H 2 S at boiling contained a few per 

 cent of bright orange-yellow globules '004 to -007 mm in diam- 

 eter. These were cell-like, having a more highly refracting 

 wall about -001 mm thick. 



Another precipitate made by heating for 2 days at 200°, 2 g. 

 of amorphous CdS in a closed tube with 10 per cent HC1, was 

 covered with a very thin film of indefinitely doubly refracting 

 material having a much redder color than any other prepara- 

 tion examined. 



