President's Address. 21 



phlogistians, however, got over this. They had proposed a 

 ridiculous theory, and they were quite ready to support it by 

 equally ridiculous assertions. Phlogiston, they explained, 

 was a principle of levity, and conferred the property of light- 

 ness upon substances with which it was combined, much, 

 indeed, in the same way as bladders lighten the swimmer to 

 whom they are attached. Although this theory found many 

 supporters even among men of genius, as witness the great 

 Henry Cavendish, it must not be supposed that it was allowed 

 to pass unchallenged. On the contrary, it was assailed from 

 many different quarters, and notwithstanding its numerous 

 friends, it ere long began to wane. Phlogiston was a sub- 

 stance so purely hypothetical, that it now became necessary, 

 in order to support the tottering theory, to produce it in a 

 tangible form, and as a last resource, hydrogen gas, with 

 which Cavendish had recently been experimenting, was de- 

 clared to be phlogiston. This was, however, an unfortunate 

 step. So long as the phlogistians confined themselves to 

 generalities, they were tolerably safe; but when they parti- 

 cularised in this decided manner, they stepped on dangerous 

 ground. The phlogiston, as described by Stahl, was found to 

 be so different from the hydrogen of Cavendish, that the theory 

 by this false stroke received its death-blow. Lavoisier, about 

 a hundred years afterwards, in a way to be shortly pointed 

 out, showed the utterly untenable nature of Stahl's doctrine. 

 The time which elapsed between the date of the enuncia- 

 tion of the phlogistic theory, and that of its final quietus, was 

 one of great progress. We are now at a point in our history 

 when we, just emerging from a period of deep gloom, dis- 

 pelled only occasionally by the results of the labours of such 

 men as Paracelsus, Helmont, Boyle, Hooke, and Mayow, are 

 about to enter one of dazzling brilliancy. The latter half of last 

 century was the period in which the foundations of modern 

 chemistry were fully laid ; and prominent among the workers 

 in this field during this fruitful time was Joseph Black, who 

 was born of Scottish parents at Bordeaux in 1728. Black's 

 great work, and the one which has made his name immortal, 

 was the discovery of latent heat, the principles of which he 

 expounded to his students in Edinburgh and Glasgow. The 



