66 
PROFESSOR GRAHAM ON THE CONSTITUTION OF 
This phosphate fuses at a red heat, after it becomes anhydrous, but it continues 
soluble in dilute acids. 
2 . Tribasic Arseniate of Magnesia and Water. ( Arseniate of Magnesia.) 
Mg- 2 H As + 2 H -f 12 H. 
This salt precipitated on mixing- dilute solutions of 500 grains of arseniate of soda 
and 300 grains of sulphate of magnesia. It consisted of 
Theory of Mg 2 H As + 14 H. 
Anhydrous salt . . . 100‘ 100- 
Water 86-53 86-25 
186-58 186-25 
This salt contains in all fifteen atoms of water, of which three are retained and 
twelve expelled at the boiling point of water. Dried at 212° it consisted of 
Theory of Mg 2 H As -f 2 H. 
Anhydrous salt . . . 100- 100- 
Water ..... 17-17 17-25 
117-17 11725 
It therefore retains pretty strongly two atoms of water besides its basic atom, resem- 
bling the preceding salt in this respect. 
This arseniate and the corresponding phosphate are rendered insoluble in dilute 
acids by the effect of a strong red heat. 
3 . Tribasic Phosphate of Magnesia and Water. {Phosphate of Magnesia.) 
Mg 2 HP + 2H + 12 H. 
This salt appears in distinct prismatic crystals in the course of twenty-four hours, 
upon mixing two ounces of sulphate of magnesia with three ounces phosphate of soda, 
each dissolved in two pounds of water. Cold water is capable of dissolving about 
one thousandth part of its weight of these crystals. They have been stated erro- 
neously to be much more soluble. The proportion of water which they contain has 
hitherto been stated at fourteen atoms instead of fifteen, which is the truth. By ana- 
lysis the crystals were found to consist of 
Theory of Mg 2 H P + 14 H. 
Anhydrous salt . . . 100' 100- 
Water 121-7 119-76 
221-7 219-76 
I find that the proportion of water retained by this salt is readily reduced at 212°, 
from fifteen atoms to seven, by the escape of eight atoms of water. Of the seven 
atoms retained one is basic, and therefore expelled with difficulty ; but from a variety 
