Effect of Temperature, Acidity, etc. 413 



the bomb was removed from the furnace and allowed to cool. 

 The glass tube was removed and opened, the volume of solution 

 remaining in the platinum tube was measured, the zinc sulphide 

 filtered off and the final acidity determined as described in the 

 chapter on marcasite. When sulphur was found mixed with 

 the sulphide, it was removed by carbon disulphide before 

 microscopic examination. 



The wurtzite from these solutions never appeared in distinct 

 crystals, but in radial parallel or felty aggregates of minute 

 prisms. The sphalerite formed clear faceted crystals which 

 were usually grouped, but rarely in single elongated dode- 

 cahedra. Regular groups usually appeared at first sight to be 

 single, deeply cross-striated, hexagonal prisms up to 0*2 mm 

 long, but further study indicated that they were strings of 

 dodecahedra twinned after the spinel law. Some regularly 

 branching groups were observed, made up of such prismatic 

 forms all pointing outward. 



Since sulphuric acid, even when dilute, is reduced by hydro- 

 gen sulphide at the temperatures of these experiments, an 

 effort was made to use solutions of zinc chloride, acidified with 

 hydrochloric acid. Although many experiments were tried at 

 temperatures as high as 350°, crystalline zinc sulphide was 

 never obtained from chloride solutions. 



The necessity of using a double tube and sulphate solutions 

 made it impossible to determine accurately the acid concentra- 

 tion at the time the zinc sulphide was being precipitated ; the 

 sulphuric acid was not only reduced by hydrogen sulphide but 

 also distilled in considerable quantity from the inside to the 

 outside tube. It was of course possible, however, to determine 

 with accuracy the acid which remained at the end of the 

 experiments, and this final acid concentration proved to be the 

 factor which determined the crystalline forms of the products 

 at any given temperature. 



Although a discussion of the distillation and reduction of 

 sulphuric acid is somewhat of a digression from the main pur- 

 pose of this paper, it is introduced because it was a necessary 

 part of the work and the results obtained are of considerable 

 chemical interest. 



B. Reduction of dilute sulphuric acid by hydrogen 

 sulphide.— The reduction of dilute sulphuric acid by hydrogen 

 sulphide was proven by the universal occurrence of sulphur in 

 the products, and the formation of sulphur dioxide in many 

 cases. This sulphur might be regarded as the product of a 

 secondary reaction of sulphuric acid on sodium thiosulphate 

 when that reagent was used, but it also appeared where 

 ammonium thiocyanate was substituted for the thiosulphate, as 

 well as where the sulphuric acid solutions were heated directly 



