296 Van Name and Bosworth — Silver Sulphate, etc. 



Mitscherlich* for silver sulphate. The silver clichromate was 

 found to be triclinic, as stated by Tschermacherf and Schabus.J 



From the above results it is clear that silver sulphate is 

 "insoluble" in the ordinary triclinic silver dichromate and 

 that the formation of mixed crystals is due to a rather limited 

 solubility of silver dichromate in orthorhombic silver sulphate. 

 The extent of this solubility at 25°, i. e. the composition of 

 the saturated mixed crystals, most be very near the value 

 given by the analysis of the crystals in experiment III, since 

 the pure silver dichromate mingled with the mixed crystals 

 was present in too small a quantity to seriously affect the 

 result, and would, moreover, tend to compensate for the error 

 introduced by the gradual change in the composition of the 

 solution during the crystallization. As a check upon this 

 result an analysis was made of another sample of approximately 

 saturated mixed crystals prepared in a different experiment. 

 The value obtained was 4*4 molecular per cent of dichromate, 

 against 4*1 per cent in experiment III. On account of the 

 small amount of material available for this last analysis, the 

 result is probably less accurate than that obtained in experi- 

 ment III. 



To determine the effect of an increase in acidity a series of 

 crystallizations was made at 25° from solutions whose total 

 acidity was approximately twice equivalent normal, prepared 

 by mixing standard solutions of the two acids in a known ratio. 

 The crystals were not analyzed as the two kinds could readily 

 be distinguished by their color. An initial ratio of 1*2 equiv- 

 alents of Cr 2 7 to 98-8 of S0 4 gave both dichromate and mixed 

 crystals, a ratio of 1*1 to 98*9, mixed crystals only. The com- 

 position of the solution which is in equilibrium with both kinds 

 of crystals must evidently lie between these limits. 



The above facts may be briefly summarized as follows : 

 The mixed crystals formed at 25° resemble the orthorhombic 

 silver sulphate in crystal form and habit, and contain a maxi- 

 mum of about 4 molecules of dichromate to 96 of sulphate. 

 The solution which is in equilibrium with these saturated 

 mixed crystals, and at the same time with pure silver dichro- 

 mate, has a composition, expressed in molecular per cent of 

 Cr 2 7 (equivalents of Or 2 7 per 100 equivalents of the mixture 

 of Cr 2 7 and S0 4 ), lying between the limits 0*90 and 0*95 when 

 the total acidity of the solution is one fourth normal, and 

 between 1*1 and 1*2 when the acidity is twice normal. 



The authors wish to express their thanks to Professor Edward 

 S. Dana for his kind assistance in the crystallographic and 

 optical tests. 



*Pogg. Ann., xii, 137, 1828. \ Phil. Mag., [2], i, 345, 1827. 



X " Bestimmung der Krystallgestalten in chemischen Laboratorien erzeug- 

 ter Producte," Wien, 1855, page 185. 



