416 Allen, Crenshaw, and Merwin — 



crystal form of the zinc sulphide at any given temperature, we 

 found this plot useful in ascertaining what initial concentration 

 to choose for any desired final concentration. 



D. The precipitation which takes place before the tube 

 reaches its final temperature, — In the case of dilute acids it 

 was impossible to prevent some precipitation of zinc sulphide 

 during the time required for heating the tubes to the desired 

 temperatures, but this was reduced to a minimum by having 

 the furnaces heated to the necessary temperature before the 

 bombs were introduced. The time required for the bomb to 

 reach the temperature of the furnace is graphically shown in 

 fig. 2. . 



A very small amount of precipitate was formed below 250° 

 since below this temperature H 2 S is not formed very rapidly 

 from the reagents in the outside tube. It is also true that 

 zinc sulphide precipitated below 250° is amorphous and easily 

 recognized by the microscope. In experiments done at 

 250°, therefore, the time required for heating the bomb 

 may be disregarded since we are concerned only with the 

 crystalline portion of the product. At 300° and 325° how- 

 ever, when the acid concentration is low, a considerable 

 amount of zinc sulphide is precipitated between 250° and the 

 final temperature, and we should therefore find relatively too 

 much wurtzite in the products, since with falling temperature 

 and constant acidity the proportion of this crystalline form 

 increases (see pp. 421-426). Experimental evidence confirms 

 this (see p. 420). If the initial acid concentration is fairly high 

 (over 3 per cent) practically no precipitation occurs before the 

 maximum temperature is reached. 



E. The Effect of Changing Conditions other than 



Acid Concentration. 



1. Hydrogen Sulphide pressure. — Before the above serious 

 variation to which the ver}' important condition — acid con- 

 centration — was subjected, was fully realized, some efforts 

 were made to control the pressure of hydrogen sulphide. This 

 was done by varying the quantity of reagent, either sodium 

 thiosulphate* or ammonium thiocyanate,f which yields the 

 hydrogen sulphide. It proved to be a physical impossibility 

 to vary the pressure greatly without varying the acid con- 

 centration greatly ; to judge from the pressure manifested 

 when the tubes were opened, it varied a number of atmospheres, 

 but in these experiments there was found no appreciable 

 influence on the crystalline form which could not be directly 

 ascribed to the acid concentration. 



* Allen, Crenshaw and Johnston, loc. cit., p. 186. 

 f Weinschenk, Zs. Kryst., xvii, 495, 1890. 



