NITROUS ACIDS. 461 



oxygen, it consists obviously of 4 equivalents of oxygen 32, and 1 of 

 nitrogen 14, which make the number representing it, 46. 



5. USES. Most of the acids of this class used in chemistry and the 

 arts, and even in medicine, are, as already stated, rather nitrous than 

 nitric acids, or rather mixtures of the two or even three varieties.* 



In medicine, this fact is of no moment, because the acid is always 

 given largely diluted with water, and in this state, it is a weak nitric 

 acid ; and indeed, in most of the arts, it is used in a state of dilution. 

 For the purposes of oxidation and combustion, the nitrous acids are 

 used indiscriminately with the nitric ; and the highly fuming acids, if 

 equally concentrated, are thought to be even better for some brilliant 

 experiments, such as the combustion of oils, of charcoal and of phos- 

 phorus. In chemical analysis, the nitric acid is generally employed ; 

 the nitrous is resorted to only occasionally. 



APPENDIX TO THE HISTORY OF THE NITROUS ACIDS. 



1 . Application of nitric oxide gas. It is obvious, from the pre- 

 ceding statements, that nitric oxide gas and oxygen gas, are mutu- 

 ally tests. To know whether there is in any gas a mixture of free 

 oxygen or of common air, it is necessary only to add a little nitric ox- 

 ide, when, if there is any uncombined oxygen gas present, the red 

 fumes will appear. So far as this fact goes, the nitric oxide is an im- 

 portant agent in the hands of the chemist, but, as regards the amount of 

 oxygen present, there has been much diversity in the results obtained 

 in different modes of operating. As the causes of this diversity could 

 not be fully understood until we had become acquainted with the ni- 

 trous acids, this subject has been reserved for the present place. 



2. Common air. When, in a glass receiver over ivater, nitric oxide 



fas is mixed with common air, the red fumes appear, and by ming- 

 ng them in proper proportions and in a proper manner, the ivhole of 

 the oxygen will be withdrawn, and the nitrogen will be left the ni- 

 trous acid being absorbed by the water. 



3. Oxygen gas. In the same manner, oxygen gas ivill be absorb- 

 ed only with more energy, and it can be known in either case, which 

 gas is in excess, by adding cautiously and in small quantities, either 

 oxygen gas or nitric oxide ; if there is a residuum of either gas, there 

 will be red fumes, on adding the other. If pure oxygen gas is em- 

 ployed, and pure nitric oxide, in proper proportions over water, the 

 absorption will be entire, and either gas, by adding the other, can 

 be completely withdrawn from any mixture of gases. 



* They are generally, described as nitric acid, holding in solution variable quan- 

 tifies of nitric oxide gas ; but the more correct view appears to be that in the text 

 (and on p. 457 d.) I have always found that the fumes obtained by heating or dilu- 

 ting the colored and fuming acids, are still more red and fuming, and indeed, it seems 

 impossible that nitric oxide gas, should be in contact with nitric acid, without de- 

 composing it, and taking enough of its oxygen, both to form and to leave nitrous acid, 

 and the same effect will of course be produced by any combustible body. 



