314 Mr Haidinger on the Cj-ystall'me Forms 



The water here is obviously identical with that which I fori 

 merly obtained from the arseniate of soda, freed from mecha^ 

 nical water by being pounded and dried in blotting-paper : 

 namely, 



Water expelled by sand-bath, - - 15.732 



Water expelled by red heat, - - - 1.162 



Dry salt left, - - - - 22.2J^6 



The water, by computation, should be 15.74! and 1.13. 



August nth, 1827. 



Art. XX. — On the Crystalline Forms of Pyrophosphate of 

 Soda and the Arseniate ofSoda, described in the preceding 

 Papers. By William Haidinger, Esq. F. R. S. E., 

 &c. Communicated by the Author. 



The examination of the regular forms of the following salts 

 I undertook at the request of Mr Clark, who put into my 

 hands the interesting substances to which the descriptions re- 

 fer. 



1. Pyrophosphate of Soda. 

 Fundamental form, a scalene four-sided pyramid. P = 



I ^go 2/ 1 , 130^ 4r, 137° 0'. Fig. 8, Plate III. Inclina- 

 tion of the axis in the plane of the long diagonal = 21° 48'. 

 Plane angles of the base 50° 8^, and 129° 52^. 

 a:b: c : d= 2.5 : 2.35 : 1.1 : 1. 

 Combinations usually like Fig. 7, whose crystallographic 



sign is p— cx)r«> ?(P). Prr^;.— Jr^;.— |r^;. 



Pr + 00 (h). Fig. 9 is the projection of it upon a plane pa- 

 rallel to the plane of inclination. Inclination of 



aonb =111° 48' a on c =103^24' 



a on e (adjacent) = 118° 22' 6 on P = 121 ° 43' 



bone = 129° 5(y b on c = 107° SC 



aonP =119° 36^ b on d{p\exP) = 101° 5P 



aond = 123° 33' 



