62 
PROFESSOR GRAHAM ON THE CONSTITUTION OF 
which approaches very closely to 6*90 grains, the number representing five atomic 
proportions of water. 
This single atom of water retained by the nitrate of magnesia, is not displaced and 
expelled upon heating the salt, together with an atomic proportion of nitrate of potash 
to 600° or 700° Fahr., so that the retention of an atom of water does not indicate a 
disposition, upon the part of nitrate of magnesia, to form a double salt. It is pro- 
bable that this peculiar and intimate combination of nitrate of magnesia with one 
atom water does not exist in the crystals or ordinary hydrate of nitrate of magnesia, 
but is the result of a new arrangement of the constituents of the salt at a high tem- 
perature. There are indications of the existence of a similar nitrate of water. 
There does not appear to be a subnitrate of magnesia like the subnitrate of copper. 
Supposed Double Nitrates and Supernitrates. 
As double nitrates are said to exist, I have repeatedly attempted to form them ; 
but when nitrate of magnesia, nitrate of zinc, or nitrate of copper was mixed with 
nitrate of potash or with nitrate of ammonia, the salts uniformly separated again in 
crystallizing. There is no proof of the existence of a single supernitrate. 
Most of the nitrates of oxides not belonging to the magnesian class are anhydrous 
salts, such as the nitrates of potash, soda, barytes, strontian, lead, &e., and do not 
suggest any new subject matter of inquiry. 
III. Of Phosphates. 
In the present state of our knowledge phosphoric acid is quite peculiar in being 
capable of combining with bases in three different proportions, forming, besides the 
usual class of salts containing one atom of acid to one atom of protoxide as base, 
two other anormal classes of salts, in which two and three atoms of base are united to 
one atom of acid, namely, the pyrophosphates and the common phosphates. Arsenic 
acid forms only one class of salts, but that class is anormal, every member of it con- 
taining three atoms of base to one atom of acid, like the common phosphates. These 
anormal classes of phosphates and arseniates, with perhaps the phosphites, are, I be- 
lieve, the only known salts to which the ordinary idea of a subsalt is truly applicable; 
or in the formulae of these salts only, ought more than one atom of any protoxide to 
appear in a basic relation to one atom of acid. All other reputed subsalts are pro- 
bably neutral in composition, as I have endeavoured to show in the case of the sub- 
nitrate of copper ; for to this salt they all bear an analogy in their small solubility 
and other properties, while they exhibit little resemblance to those classes of phos- 
phates and arseniates which really possess more than one atom of base. The fol- 
lowing Table contains the formulse of the most important phosphates, with a new 
nomenclature of these salts, which I offer for consideration. 
