38 ESSAYS BIOGRAPHICAL AND CHEMICAL 



that at that time, the current doctrine was that when 

 substances burn, they lost a principle, to which the name 

 ' phlogiston ' had been applied by Stahl, the propounder of 

 the doctrine. The hydrogen evolved was at first sup- 

 posed by Cavendish to be the long-sought phlogiston 

 itself. But fuller consideration induced him to change 

 his view ; and he subsequently held that hydrogen was a 

 hydrate of phlogiston, or a compound of that hypothetical 

 substance with water. In this paper, too, as well as in 

 one which followed, Cavendish added many facts to those 

 which had been published by Black on the properties of 

 carbonic acid; but as these contain little of theoretical 

 interest, they need not detain us. 



Seventeen years later, the next of his ' pneumatic ' 

 papers was published. It was entitled, ' An Account of a 

 New Eudiometer.' The eudiometer, which in no way 

 resembled the picture of the instrument usually ascribed 

 to him, was designed, not for the explosion of a mixture 

 of two gases, but for the removal of oxygen from air, by 

 means of nitric oxide. With its aid, he determined the 

 composition of many samples of air, and his final result, 

 translated into our method of statement, gave for the 

 proportion of oxygen in air the extraordinarily accurate 

 number, 20*83 per cent. 



Cavendish's next paper in order of publication (1784) 

 gave the results of experiments begun in 1781. Its title 

 is ' Experiments on Air.' The object of these experiments 

 was to 'find out the cause of the diminution which com- 

 mon air is well known to suffer by all the various ways 

 in which it is phlogisticated, and to discover what becomes 

 of the air thus lost or condensed.' His first idea was that 

 this treatment might result in the formation of ' fixed 

 air.' But having disproved this, he proceeded to try 

 whether, as some of Priestley's experiments appeared to 

 show, ' the dephlogisticated part of common air might 



