144 
was then concentrated, and the strongest nitric acid (boiling 
at 86° C) added until the calcium nitrate commenced to 
separate out. This was immediately redissolved by the 
addition of a few drops of dilute nitric acid. The nitric 
acid solution is now in the highest possible state of concen- 
tration : on throwing a small quantity of tin into this 
solution, the metal is rapidly oxidized to stannic acid, and 
the supernatant liquid remains perfectly clear. The pre- 
liminary heating of the solution is indispensable, since in 
the cold the metal is apt to become passive, when it com- 
pletely resists the action of the acid. The precipitate is 
now dissolved in a small quantity of caustic potash, and 
saturated with hydrogen sulphide : on adding acetic acid in 
slight excess the tin sulphide is precipitated. The preci- 
pitate is then separated by means of the Bunsen '■‘filter- 
pump,” and the whole of the phosphoric acid is contained in 
the filtrate. After concentrating the solution, and again 
filtering from a minute precipitate of tin sulphide, which 
invariably separates out (tin sulphide being slightly soluble 
in solutions containing hydrogen sulphide), the phosphoric 
acid may be precipitated as the magnesium ammonium 
salt, and weighed as pyrophosphate. 
I. Katio of tin to phosphoric acid, 4 to 1. 
1. 0*5135 grm. cal. phos. gave 0*405 Mg 2 P 2 0 7 = 50*4 5°/ 0 P 2 0 5 
II. 0*447 „ „ 0*358 „ 50*78 „ 
Mean 50*61% P 2 0 5 
The lime was determined as caustic lime after removal of 
the lead by means of hydrogen sulphide. The mean of two 
concordant analyses gave 49*65% lime. 
Hence the composition of the calcium phosphate is 
Phosphoric Acid 50*61 
Lime 49*65 
100-26 
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