AIR. 
evident, that in all cases of a mixture of these two 
gases, the diminution will be proportional to the quan 
tity of the oxygen. Of course it will indicate the pro 
portion of oxygen in air ; and by mixing it with diffe 
rent portions of air, it will indicate the different quan 
tities of oxygen which they contain, provided the com 
ponent parts of air be susceptible of variation. Dr 
Priestley’s method was to mix together equal bulks of 
air and nitrous gas in a low jar, and then to transfer 
the mixture into a narrow graduated glass tube about 
three feet long, in order to measure the diminution 
of bulk. He expressed this diminution by the number 
of hundred parts remaining. Thus, suppose he had 
mixed together equal parts of nitrous gas and air, the 
sum total of this mixture was 200 (or 2.00): suppose 
the residuum when measured in the graduated tube to 
amount to 104 (or 1.04), and of course that 96 parts 
of the whole had disappeared, he denoted the purity of 
the air thus tried by 104. A more convenient instru 
ment was invented by Dr Falconer of Bath; and Fon 
tana greatly improved this method of measuring the 
purity of air. A description of his eudiometer was pub 
lished by Ingenhoutz in the first volume of his Experi 
ments; it was still farther improved by Cavendish in 
1783*, and Humbolt has lately made a very laborious 
set of experiments in order to bring it to a state of 
complete accuracy. But after all the exertions of these 
philosophers, the method of analysing air by means of 
nitrous gas is liable to so many anomalies, that it can 
not be depended on. 
Priestley and Fontana have proved, that the way of 
* Phil. Trans. lxxiii. 107. 
R 2 
259 
Chap. I.