PRIESTLEY. 77 



combination which nitrous gas forms suddenly with 

 oxygen ; diminishing the volume of both in proportion 

 to that combination ; and he thus invented the method 

 of eudiometry, or the ascertainment of the relative 

 purity of different kinds of atmospheric air. 



It must not be forgotten, in considering the great 

 merits of Priestley as an experimentalist, that he had 

 almost to create the apparatus by which his processes 

 were to be performed. He, for the most part, had to 

 construct his instruments with his own hands, or if he 

 employed others, he had to make unskilful workmen 

 form them under his own immediate direction. His 

 apparatus, however, and his contrivances for collecting, 

 keeping, transferring gaseous bodies, and for exposing 

 substances to their action, were simple and effectual, 

 and they continue to be still used by chemical philo- 

 sophers without any material improvement. It was 

 fortunate in this respect that he began his pneu- 

 matic inquiries with seeking for the means of im- 

 pregnating water with carbonic acid; this inquiry 

 naturally turned his attention to the contrivance of 

 apparatus and generally of manipulations, serviceable 

 in the examination of bodies whose invisible form and 

 elastic state renders inapplicable to them the machinery 

 of the old laboratory, calculated only for solids and 

 liquids. 



The pertinacity with which Priestley clung to the 

 phlogistic theory is marvellous. It might have been 

 expected, that the fact of a combustion leaving the 

 residue, whether of two gases, or of a gas and an in- 

 flammable body, exactly equal in weight to the sum 

 of the weights of the bodies burnt and which had dis- 

 appeared in the process, would have been accepted as 

 a proof that these two bodies had entered into an 

 union, giving out the latent heat which had previously 

 held the gaseous body or bodies in a state of aeriform 

 fluidity. It might, in like manner, have been ex- 

 pected, that when a metal, by absorbing oxygen gas, 



