249 
Mr. Pepys* Account of a new Eudiometer. 
that portion of atmospheric air which he calls dephlogisticated , 
first proposed its being used as a test for ascertaining the purity 
of air. His method of proceeding was ingenious and simple ; 
known quantities of the air to be tried, and of nitrous gas being 
mixed, were admitted after the diminution of volume occasi- 
oned by their union, into a graduated tube, which he denomi- 
nated a eudiometer. 
It was with the test of nitrous gas that Mr. Cavendish* 
made his masterly analysis of the air at Kensington and 
London ; and by many laborious processes and comparative 
trials obtained results, the accuracy of which has been more 
distinctly perceived the more the science of chemistry has 
advanced. 
The slow combustion of phosphorus, which unites with the 
oxigene to form an acid, and the decomposition of the fluid 
sulphuret of potash, are certain methods of separating combi- 
nations consisting of oxigene and azote : but the decomposition 
is effected so slowly, by the action of these substances, that 
it became a desirable object, to discover some means for ac- 
celerating the process. This was supposed to have been ef- 
fected by Guyton, who proposed heating the sulphuret of 
potash ; in doing this, sulphurated hydrogene gas however is 
frequently evolved, which, mixing with the residual gas, in- 
creases its quantity, and renders the result fallacious. 
The green sulphate of iron impregnated with nitrous gas, 
first discovered by Dr. Priestly, and recently used by Mr. 
Davy for eudiometrical purposes, from its possessing the pro- 
perty of absorbing oxigene gas from the atmosphere, is much 
* Phil. Trans, for 1783. 
