10 



reasoning as this, some of the best chemists of the day were misled, not 

 only as to the direction of their labours, but even the results of their ex- 

 periments. But in Cavendish's celebrated inquiry into the causes where- 

 by air suffers diminution in a variety of processes then termed phlogistic, 

 it is well worthy of remark how steadily he moves on from truth to truth, 

 on every point on which experiments afforded ground for reasoning, un- 

 fettered by the complexity of the phlogistic theory ; and it is equally 

 remarkable, how loose he sits to the favourite hypothesis to which the 

 rest of his countrymen clung with such persevering tenacity. He, 

 first of all his contemporaries, did justice to the rival theory recently 

 proposed by Lavoisier, and weighed it in equal scales before the pub- 

 lic eye. He alone seemed to understand, as it became a disciple of 

 the school of Newton, the true use of a hypothesis : he valued neither 

 system otherwise than as an expression of facts, or as a guide to future 

 inquiry. He took these opposite hypotheses, and retrenched their su- 

 perfluities ; he pared off from both, their theories of combustion, and 

 their affinities of imponderable for ponderable matter, as complicating 

 chemical with physical considerations ; and he then corrected and ad- 

 j usted them with admirable skill to the actual phaenomena, not bending 

 the facts to the theory, but adapting the theory to the facts. 



Allow me to give you an instance of this adaptation. Priestley had 

 stated, that he had converted charcoal into inflammable gas by the sim- 

 ple action of the burning lens, and obtained it from pure iron, by the 

 same means, and had drawn the consequence, that iron was composed 

 of phlogiston united to the basis, or calx, of iron, and that charcoal and 

 inflammable gas were pure phlogiston. " I had no suspicion," he says*, 

 " that water was any part of inflammable air ;" " yet that water in great 

 quantities is sometimes produced from burning inflammable and de- 

 phlogisticated air, seemed to be evident from the experiments of Mr. 

 Cavendish and M. Lavoisiei'. I have also frequently collected consi- 

 derable quantities of water in this way, though never quite so much as 

 the two kinds of air decomposed." " Afterwards, seeing much water 

 produced in some experiments in which inflammable air was decom- 

 posed, I was particularly led to reflect on the relation which they bore 

 to each other, and especially Mr. Cavendish's ideas on the subject. He 

 had told me, notwithstanding my former experiments, from which I had 

 concluded that inflammable air was pure phlogiston, he was persuaded 

 that water was essential to the production of it, and even entered into 

 it as a constituent principle. At that time I did not perceive the force 



* Priestley on Air, ed. 1790, part 3. sect. 4. 



