PURIFICATION OP NITRIC ACID. 
119 
of M. Millon, this concentrated acid contains exactly one 
equivalent of water, and represents the first degree of hy- 
dratation of nitric acid. According to calculation, the 
equivalent would be 14.24 to 100. Experience gives 15.07 
for an acid, the density of which is 1.521 at 50° Fahr. 
It has been generally admitted that concentrated nitric 
acid enters into ebullition at 187° Fahr. According to M. 
Millon, this appears not to be a fixed point, being retained 
only for a few moments by all the acids of a greater density 
than 1.50, 194°, 222°, and 232° Fahr., are analogous points, 
which occur, according to the quantity of the liquid, its 
density, the application of the heat, and several other cir- 
cumstances, without the acid being fixed in its constitution. 
All the others contain a considerable quantity of nitrous 
acid, of which the heat alone does not deprive them. Thus 
nitric acid comes over in distillation mixed with nitrous 
acid, and its boiling point rises until it arrives at 253° Fahr. 
The acid which subsequently distils is colorless, and causes 
no precipitation with sulphuretted hydrogen. Its density 
is 1.484 at 64° Fahr.; this is an acid with two equivalents 
of water. It is only obtained to any extent in operating 
upon a large quantity of very concentrated acid. Seventeen 
pints afford about one pint. If the distillation be continued, 
the acid becomes more and more weak, until it arrives at 
that state of hydratation, at which it possesses considerable 
stability: it then contains four equivalents of water. 
The general law relating to this distillation, has been 
well characterized by Dalton, who established one prin- 
ciple — that concentrated nitric acid becomes progressively 
weaker by distillation, whilst a weak acid is rendered 
stronger. M. Millon has observed, however, that the form 
of the retort in which the acid is distilled, and the presence 
of platinum wire, are capable of changing the point of ebul- 
lition, and even the nature of the product. In fact, in dis- 
tilling a weak acid, without platinum wire, at a temperature 
from 257° to 262° Fahr., while the temperature was rapidly 
