On Crystalline Form and Chemical Proportions. 201 
in the crystallized arseniates or phosphates is consequently 
different from that in which they exist dissolved in water. If 
the arseniate and phosphate of soda crystallize in a solution 
which already appears alkaline to re-agents, the liquor, after 
the crystallization, is manifestly acid. If, on the contrary, the 
base of the crystallizing salt is potash, the liquor shews a 
highly alkaline state. In the first case, the salt which crystal- 
lizes is aneutral salt; in the second, itis an acid salt. Ifwe 
moisten litmus paper with the solution of bi-phosphate or bin- 
arseniate of potash, the paper becomes red; but on drying, the 
blue colour returns, for the salt resumes, on crystallizing, the 
quantity of acid which had reddened the test paper. 
It is, in general, very easy to determine the degree of satura- 
tion of an arseniate or phosphate. The neutral salts precipitate 
the muriate of barytes, so that the liquor shews a neutral re- 
action, and a neutral salt is precipitated. It is, however, to be 
remarked, that the neutral arseniate and phosphate of barytes 
are, in a very slight degree, soluble in water, and that we ought 
never to precipitate the arseniates or phosphates by muriate of 
barytes, because that then the excess of the base, which is free 
in the solution, would convert the neutral phosphate and arse- 
niate of barytes into a a subsalt; and hence, for the same rea- 
son, we ought to add the arseniate or phosphate only drop by 
drop to the muriate of barytes. The neutral solutions of oxide 
of lead comport themselves in the same manner with the neutral 
phosphates and arseniates. The bin-arseniates and bi-phos- 
phates do not precipitate the solutions of muriate of barytes ; 
but on adding the smallest quantity of ammonia, or of any 
other soluble base, we obtain a precipitate. Another very ready 
mode of ascertaining the degree of saturation of the phosphates 
and arseniates, with base of soda and potash, is to heat them 
to redness with the carbonate of potash or soda. They expel 
carbonic acid till a subsalt is formed, in which subsalt the oxy- 
gen of the base is to that of the acid as 1 to 14; and from the 
loss of carbonic acid we calculate the quantity of arsenic or 
phosphoric acid which the salt contains. 
A perfectly exact analysis of the phosphates and arseniates, 
presents almost insurmountable difficulties. 
Of the Bin-arseniate aud Bi-phosphate of Potash. 
The bin-arseniate of potash occurs in commerce in large 
crystals. It is manufactured in Saxony and other places, by 
fusing one part of nitre with one part of arsenious acid. The 
melted mass is dissolved in water, from which crystals are ob- 
tained, after evaporation. The best manner of preparing these 
two salts is to add carbonate of potash to phosphoric or arsenic 
acid, till litmus paper reddened by the solution, becomes blue 
