366 Allen and Crenshaw — Sulphides of Zinc, 



amorphous or crystalline nature of the substance and on its 

 state of division; and in a minor degree on the nature of its 

 surface, etc. 



1st. Tlu' crystals vary from clear yellow, in tufts of hair-like 

 needles, to brownish yellow in the larger ones, but the powder 

 of all specimt ns is orange. Still, there is no doubt that all arc 

 the same crystalline form ; the different colors are due to the 

 relative amounts of light transmitted and reflected. 



2d. Amorphous cadmium sulphide shows all tints from 

 lemon yellow to orange red. Light yellow products are 

 obtained by precipitating cold solutions of low cadmium con- 

 centration by hydrogen sulphide, or by precipitating cadmium 

 solutions with the alkali sulphides. The microscope shows 

 that the globules of these amorphous precipitates are all very 

 small and of similar magnitude. 



The deep orange precipitates are obtained by precipitating 

 hot acid cadmium solutions with hydrogen sulphide, or by long 

 boiling of cadmium solutions with excess of thiosulphate. 

 When the first method is used, a high concentration of acid 

 gives a deeper-colored product. The orange amorphous cad- 

 mium sulphide is made up of larger aggregates than the yellow 

 — sometimes fifty times as great in diameter. (See micro- 

 scopic part.) A very illuminating experiment on this subject 

 was carried out as follows : To about 200 cc. water 4 g. NaCl, 

 10 g. Na 2 S,0 3 .5H 2 and 2 g. CdS0 4 .8/3H,0 were added. The 

 solution was heated to boiling and kept in ebullition for some 

 hours, hot water being added from time to time. Precipita- 

 tion began almost at once. After about an hour a little of the 

 lemon yellow product was removed and examined micro- 

 scopically. The particles were too small to measure. After 

 2w hours the product had much increased in quantity, but was 

 still light yellow. The microscope showed that it was made 

 up of clear globules about •001 mm in diameter. Gradually the 

 precipitate grew denser and darker in color until after 4 or 

 5 hours it had all settled to the bottom, and showed a bright 

 orange color. This product contained particles of all sizes up 

 to -05 mm in diameter. 



Another piece of evidence in point is this : the light yellow 

 precipitate formed by the action of ammonium sulphide on a 

 cadmium salt retains the same color after the system has been 

 heated, though the product is now all crystalline greenockite. 

 When, however, the latter is ground in a mortar it becomes 

 orange in color. This proves quite conclusively that the 

 difference in color has no connection with another crystalline 

 form. Klobukow gives a much lower density for the yellow 

 products than for the orange, viz., 3*906 — 3 - 927 against 

 4492 — 4-513. This looks like a specific constant, but it must 



