145 
It is therefore evident that the amount of pure tin re- 
quired need not exceed 4 times the weight of phosphoric 
acid present. 
The following experiments show that this is moreover 
the minimum quantity that can be used. 
II. Ratio of tin to phosphoric acid = 3*0 to 1. 
0-477 grm. cal. phos. gave 0*307 M g 2 P 2 0 7 = 4T53°/ o P 2 0 6 
III. Ratio of tin to phosphoric acid = 3*5 to 1. 
0-598 grm. cal. phos. gave 0*388 Mg 2 P 2 0 7 = 41'57 0 P20 5 
0-434 „ „ „ 0-304 „ 44-82 „ 
In order to confirm the above results I have determined 
the proportion of lime and phosphoric acid contained in the 
calcium phosphate employed in the analyses, by dissolving 
the compound in hydrochloric acid, and adding sufficient 
sulphuric acid to precipitate the base. To each volume of 
the liquid two volumes of alcohol were added, and the mix- 
ture allowed to stand about 12 hours, when it was filtered 
and the precipitate thoroughly washed with alcohol. The 
filtrate containing the phosphoric acid is evaporated to dry- 
ness, the residue dissolved in water, and the acid precipi- 
tated as the magnesium ammonium salt. The lime was 
weighed as sulphate. 
I. 0*525 grm. gave 0*411 Mg. P 2 0 7 = 50-08 °/ Q P 2 Ch 
II. 0-507 „ 0-396 „ „ -49*96 °/ o PA 
The lime amounted to 50 per cent. Hence the composition 
of the calcium phosphate is 
P 2 0 5 50-02 
CaO 50-00 
100-02 
exactly agreeing with the determinations made by the tin 
method. 
