of Pyrophosphate of Soda. 315 



Observatio7is. — The crystals have a perfect conchoidal frac- 

 ture, and are quite permanent when exposed to the air ; they 

 do not effloresce hke the common phosphate. I have often 

 seen the latter substance, but never examined- its crystallogra- 

 phic properties, this having been already done by Professor 

 Mitscherlich ; but I cannot at present turn to his description 

 of it, in order to contrast the forms of the two salts. 



% Arseniate of Soda. 

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



I ^|I ^11 , 113° 2r, 1 J 9° oQ'. Fig. 8. Inclination of the 



axis in the plane of the long diagonal = 7° 0'. Plane angles 

 of the base 78° IS^, and 101° 42'. 



a: b:c:d -8.2: 7.54: 6.14:1. 

 Combinations usually like Fig. 10, whose crystallographic 



sign is P- QD (a). J (P). Prr^;.-^J:r^>->| (c). 



P + 00 09;- (Pr + Qoy(g). Pr + 00 (b). Pr4. QD (h.) 

 Fig. 11 is a projection of it upon a plane, parallel to h. In- 

 clination of ^ 

 aonh = 97^ 0' « on c = 116° m' 

 a one (adj.) - 128° Tl' g on g (over b) = IIT W 

 eonb =1 34° 33^ ./ on f (over b) = 78° 46^ 

 a on P = 123° 22/ a onf = 94° 26' 

 Observation'^. — There is a distinct cleavage parallel to the 

 face b ; otherwise the fracture is conchoidal. This salt was 

 first described by Professor Marx of Brunswick, in a pamphlet 

 " On the Relations between the Chemical Mixtitre and the 

 Crystalline Form,''''*' and urged against Professor Mitscherlich 

 as an instance of want of similar forms in two salts, consisting 

 of similar sompounds of isomorphous bodies. Professor Leo- 

 pold Gmelin has since shown, -|- that the arseniate of soda 

 crystallizes with two different proportions of water, and pro- 

 duces, therefore, two essentially distinct species, one of them 

 being efflorescent, which is that described by Mitscherlich, while 

 the other is permanent, which was obtained by Marx. The 



• Ueber das VerhdUniss der Mischung sur Form* ISSi. 

 T Poggendorf's Annalen. 1825-6. 



