248 Mr. Pepys' Account of a new Eudiometer. 
properties altogether different, from those of the ingredients 
supposed to be present ; and the facilities which every new de- 
tection of unsuspected principles afford, towards the discovery 
of others, and consequently the composition, or analysis of 
bodies before held to be simple, it will not appear a matter of 
surprize, that the subject of eudiometry, should have obtained 
a considerable degree of attention from modern philosophers. 
This would be an improper place to enumerate all that has 
been done, or proposed, by different men of eminence, towards 
the production of something like a perfect system on this im- 
portant subject ; yet some allusion to their labours appears to 
be indispensible, and will be the means of preventing some 
circumlocution in our farther progress. 
Hales* appears to be the first who observed absorption to 
take place in common air, on mixing it with air obtained from 
a mixture of Walton pyrites and spirits of nitre; and that in 
this process, from being clear they became “ a reddish turbid 
fume / 5 
Dr. Priestly, as he informs us in his Observations on different 
kinds of Air,f was much struck with this experiment, but never 
expected to have the satisfaction of seeing this remarkable ap- 
pearance, supposing it to be peculiar to the Walton pyrites, 
till encouraged by a suggestion of Mr. Cavendish, that pro- 
bably, the red appearance of the mixture depended upon the 
spirits of nitre only, he tried solutions of the different metals 
in that acid, and catching the air which was generated, obtained 
what he wished. To the air thus produced, he gave the name 
of nitrous air, and from its possessing the properties of absorbing 
* Statical Essays, Vol. I. p. 224. Vol. II. p. 280, 
t Phil. Trans, for 177 2, p.210. 
