ON PHOSPHATE OF AMMONIA. 
15 
of lime. The supernatant fluid is~then saturated with car- 
bonate of ammonia, which combines with the excess of 
phosphoric acid, and precipitates the phosphate of lime 
which it held in solution, and the filtered liquor, after con- 
centration by a gentle heat, is set aside to crystallize. 
The evaporation of the solution should be conducted at 
a very low temperature, which at no time should exceed 
100° Fahr., and for this purpose the drying room heat is 
perhaps the most favourable. By spontaneous evaporation 
the salt is obtained in more regular and larger crystals; and 
where time is no impediment, that mode of isolating it adds 
much to the beauty of the product. The necessity of using 
a low temperature is owing to the extreme readiness with 
which the neutral phosphate parts with one half of its am- 
monia, and assumes the state of acid, or bi-phosphate, which 
change, besides diminishing the product, unfits the salt in 
great measure for producing the therapeutic effect required 
of it. After removing each crop of crystals, it is necessary to 
concentrate the solution, and to add more ammonia, so as 
to crystallize from a neutral or slightly alkaline solution. 
Neutral phosphate of ammonia crystallizes in square 
prisms, terminated with four-sided pyramids, with the apex 
truncated. Crystals of the alkaline phosphate, which as- 
sumes the form of six-sided plates, derived from the rhom- 
bohedron, are frequently found mixed with the neutral 
salt. It should be observed that the solution requires to be de- 
colorized with animal charcoal before being placed aside for 
crystallization. 
It has been suggested that oxalic acid, if added to the 
solution of super-phosphate of lime, would by its superior 
affinity for lime Ithrow down that earth, and thus increase 
the amount of phosphoric acid in the solution capable of 
uniting with the ammonia. We have not had an oppor- 
tunity of testing its power by experiment, and hence can- 
not recommend its use, but we believe, even if it should 
enjoy that property, its price, added to the danger of con- 
