294 NITROGEN. 



ness. The colourless liquid acid becomes yellow, when exposed 

 to the rays of the sun, and on loosening the stopper of the 

 bottle, it is sometimes projected with force, from the state of 

 compression of the disengaged oxygen. Hence to preserve 

 this acid colourless it must be kept in a covered bottle. It con- 

 geals at about 40, but diluted with half its weight of water, 

 it becomes solid at li, and with a little more water its freezing 

 point is again lowered to 45. Exposed to the air the concen- 

 trated acid fumes, from the condensation by its vapour of the 

 moisture in the atmosphere. It also attracts moisture from 

 damp air, and increases in weight; and when suddenly mixed 

 with 3-4ths of its weight of water, may rise in temperature from 

 60 to 140. 



Nitric acid has a great affinity for water, and diminishes in 

 density with the proportion of water added to it. A table has 

 been constructed by Dr. Ure, in which the per centage of abso- 

 lute acid is expressed in mixtures of various densities, which is 

 useful for reference and will be given in an appendix. There 

 are several definite hydrates of this acid. The most concen- 

 trated acid contains a single equivalent of water ; a second acid 

 appears to exist, having a density of about 1.48, which contains 

 two of water, and forms the nitric acid which has little or no 

 action upon tin, iron and some other metals ; there is still a third 

 acid of density 1.42, which contains four equivalents of water. 

 This last hydrate was found by Dr. Dalton to have the highest 

 boiling point of any hydrate of nitric acid, namely 248, and 

 both weaker and stronger acids are brought to this strength by 

 continued ebullition, the former losing water and the latter acid. 

 The density of the vapour of this hydrate is found to be 1243 by 

 A. Bineau, and it contains 2 volumes of nitrogen, 5 volumes of 

 oxygen and 8 volumes of steam condensed into 10 volumes, 

 which are therefore the combining measure of this vapour.* 



Nitric acid is exceedingly corrosive, and one of the strongest 

 acids, yielding only in that respect to sulphuric acid. The faci- 

 lity with which it parts with its oxygen, renders it very proper 

 for oxidating bodies in the humid way, a purpose for which it is 

 constantly employed. Nearly all the metals are oxidized by 

 means of it ; some of them with extreme violence, such as cop- 

 per, mercury and zinc, when the concentrated acid is used ; and 



* An. de Ch. et cle Ph. t. 68, p. 418. 



