418 Allen, Crenshaw, and Merwin — 



If we compare products obtained from the same final acid, 

 the initial zinc concentration varying in one series and remain- 

 ing constant (2 per cent) in the other (Tables VII and XI), we 

 shall find that more than half the results agree within the 

 limits of error of the microscopic determination. In other 

 cases the quantity of sphalerite obtained from concentrated 

 zinc solutions is greater than from dilute zinc solutions. This 

 variation, however, mav be explained if we note that in one 

 series of experiments the acid concentration is rising, in the 

 other falling (see pp. 421). 



3. Sodium Sulphate. — In the earlier stages of our work in 

 order to prevent the distillation of sulphuric acid from the 

 platinum tubes (see p. 414) sodium sulphate was introduced to 

 lower the vapor pressure. By this means it was possible to 

 keep the acid concentration much more nearly constant during 

 the course of the precipitation of the zinc sulphide, but the 

 products were usually not well crystallized and in many cases 



Table VIII. 

 Influence of Na^SO^ on the crystalline form of ZnS at 300°. 



Initial 



Final 









Crystalline 



H 2 S0 4 



H 2 S0 4 



Na 2 SO 



V 













in wt. % 



in wt. % 



grams 



Time 



% Amorphous 



% Wurtzite 



% Sphalerite 



2 



2-2 



1-0 



1 day 



60-75 



50 



50 



2 



1-9 



2-0 



2 



nearly all 



? 



? 



2 



2-1 



2-0 



1 



100 





_ . - . 



5 



3'4 



1-0 



1 



20 



? 



nearly all 



5 



3-5 



2-0 



If 



25 



67 



33 



5 



4*0 



5-0 



1 



50 



50 



50 



8 



6-2 



2-0 



1 



some 



? 



some 



10 



6-8 



2-0 



11 



15 



? 



100 



10 



7*0 



20 



1 





no ZnS 







10 



6-6 



5*0 



1 



40-50 



100 



? 



10 



7-6 



5'0 



1 



25 



100 







10 



7'5 



5-0 



1 



5 



70-8^ 



20-30 



15 



10-2 



5*0 



1 



20-30 



90-95 



5-10 



15 



11-4 



5-0 



1 



30-40 



90-95 



5-10 



20 



12'1 



5-0 



l£ 







99 



<1 



an estimate of the percentage of wurtzite could not be made. 

 The results are shown in Table VIII. Almost without excep- 

 tion the products show more sphalerite than those obtained 

 from solutions of the same final acid concentration where 

 sodium sulphate was not used. Although this may be due 

 specifically to the presence of sodium sulphate it seems more 

 probable that the variation is due to the reduction of the 



