232 The Rise of the Modern [lect. 



first in an elastic state, in which the particles repel each other, 

 and secondly in a reduced or fixed state, in which their particles 

 are attracted by the particles of some other substance, e.g. 

 sulphureous particles. 



I refer to him not because he made any definite special 

 contribution to our knowledge of respiration (though his work 

 had a remarkable practical side through the introduction of 

 ventilation), but because his writings contain the first clear 

 enunciation of the existence of gases in a free and in a com- 

 bined condition. By clearly stating this principle he exercised 

 a notable influence on other men's researches, and thus power- 

 full}^ aided the discoveries which were made by others after him. 

 This is what he says : 



"Since, then, air is found so manifestly to abound in almost 

 " all natural bodies ; since we find it so operative and active a 

 " principle in every chymical operation ; since its constituent 

 " parts are of so durable a nature, that the most violent action 

 " of fire or of fermentation cannot induce such an alteration of 

 " its texture as thereby to disqualify it from resuming either by 

 B fire or fermentation its former elastick state ; . . . . since 

 " then this is the case may we not with good reason adopt this 

 " now fixed, now volatile Proteus among the chymical principles, 

 " and that a very active one, as well as acid sulphur ? notwith- 

 standing it has hitherto been overlooked and rejected by 

 " chymists, as in no way entitled to that denomination." 



Hales, it will be observed, speaks of air (or gas) as if it were 

 always the same thing. He knew that air had not always the 

 same properties, that sometimes it was inflammable and some- 

 times not, sometimes good for breathing, sometimes not, but 

 these were instances of varying qualities of the same thing, not 

 of different things. He ignored van Helmont's discovery of a 

 gas which was a different thing from air. But the avatar of 

 the now nearly forgotten van Helmont was soon to come. 



In 1754 there appeared a Dissertatio de humore acido a 

 cibo orto et de magnesia by one Joseph Black, who, born at 

 Bordeaux in 1728, had been educated at Belfast, Glasgow, 

 and Edinburgh, and who in the year following the appearance 

 of his dissertation was appointed Professor of Chemistry at 



