v THE STUDY OF GASES 73 



Priestley studies the combination of nitrouj air with 

 common air and with oxygen. The most remarkable 

 property of nitrous air was that of combining with common 

 air to form brown NITROUS FUMES 1 which dissolved at once 

 in water. Priestley made a careful study of this action and 

 showed that the diminution of volume occasioned by the 

 addition of an equal volume of nitrous air could be used 

 to measure the goodness of common air. Speaking of 

 nitrous air he says : 



" One of the most conspicuous properties of this kind of 

 air is the great diminution of any quantity of common air 

 with which it is mixed, attended with a turbid red, or deep 

 orange colour, and a considerable heat " 



" The diminution of a mixture of this and common air is 

 not an equal diminution of both the kinds . . . but of about 

 one-fifth of the common air, and as much of the nitrous air 

 as is necessary to produce that effect ; which, as I have 

 found by many trials, is about one-half as much as the 

 original quantity of common air. . . ." 



" If, after this full saturation of common air with nitrous 

 air, more nitrous air be put to it, it makes an addition equal 

 to its own bulk, without producing the least redness, or any 

 other visible effect. . . ." 



" It is exceedingly remarkable that this effervescence and 

 diminution, occasioned by the mixture of nitrous air, is 

 peculiar to common air, or air fit for respiration and, as 

 far as I can judge, from a great number of observations, is 

 at least very nearly, if not exactly, in proportion to its fitness 

 for this purpose ; so that by this means the goodness of air 

 may be distinguished much more accurately than it can be 

 done by putting mice, or any other animals, to breathe in it" 

 (Experiments on Air, I774, 2 I. 110-115). 



1 These fumes were generally regarded as the vapour of nitric acid ; 

 Priestley described them as "nitrous acid vapour," Davy (1800) as 

 " nitrous acid gas," Gay-Lussac in 1809 as " nitric acid " and in 1816 

 as "nitrous acid"; in order to avoid confusion, the term "nitrous 

 fumes" is used in the succeeding pages. The name " nitrogen per- 

 oxide" was not introduced until about 1850. 



2 The first part of Vol. I., published in 1774, describes experiments 

 carried out in 1772. 



