On Crystalline Form and Chemical Proportions. 205 
Phosphoric acid. . ae 4 
Ammonia. . . . 25.864100. 
Watery: (dk) onbteti20734 
Of the Faces of the Crystals, and of their relative Situation. 
The primitive form of the crystals of these two salts, is an 
oblique prism with rhombic bases (Fig. 5 and 6). The mean of 
a great many measures gave for the inclination of the plane M’ 
on the plane M”, in the arseniate of ammonia 85° 54’, and in 
the phosphate of ammonia 84° 30’, The plane f replaces the 
solid angle A, and forms with P an edge parallel to the oblique 
diagonal of the plane. The inclination of f to P was, in the 
arseniate of ammonia, 109° 8’; in the phosphate 109° 44’. It 
was found on measuring also the inclination of P to M, that the 
cotangent of the angle, which P makes with the axis is in the 
two salts, to that of the angle which f makes with the axis in the 
ratio of 1 to 24. From these data M. M. deduced, that P 
makes with the axis an angle of 66° 292’ in the arseniate of am- 
monia; and an angle of 66° 45’ in the phosphate. 
On the Neutral Arseniate and Phosphate of Soda. 
We obtain these two salts, if we add carbonate of soda to 
arsenic or phosphoric acid, till the solution appear alkaline 
by test paper. As they are much more soluble in hot than 
in cold water, they crystallize readily from a concentrated 
solution. These salts precipitate the muriate of barytes so 
that the solution remains neutral; and the muriate of lime, 
so that the solution exhibits a very acid re-action. They are 
consequently in the same degree of saturation as the neutral 
arseniate and phosphate of barytes. 100 parts of arseniate 
of soda are composed of 
Arsenic Acid . 65.84 
Soda. . . . 34.16 
The arseniate of soda, the base of which contains 2! times 
the oxygen present in the acid, is consequently, according to 
the atomic weights of Berzelius, a compound of 
Arsenic Acid . 64,82 
; Badass. sii it wciple 
“100 parts of the dry salt unite with 126.3 of water; whence 
the oxygen of the base is to that of the water as 1 to 12. It 
thence follows that 100 parts of the crystals consist, by Ber- 
zelius, of Arsenic Acid . 29.29 
Sodair. silyen ailores 
Wateto ica sal) 2) 54.84" 
4.387 grains cf phosphate of soda, which did not contain a 
trace of sulphate of soda, added to muriate of barytes, gave 
* M. Mitscherlich remarks here, that Dr. Thomson must have con- 
founded the neutral salt with the bin-arseniate in his analysis. 
