20 Proceedings of the Royal Physical Society. 



phlogiston created in its day a profound impression, and 

 attracted a vast deal of attention; but I think that, in our 

 own time, it is very generally conceded that its usefulness 

 has, to say the least, been very greatly overrated; and I 

 imagine I am tolerably safe in saying, that about the only 

 real service it ever rendered, was that of enabling a certain 

 amount of method to be introduced into what had been 

 hitherto, no doubt, exceedingly unsystematic. It has been 

 said, and firmly maintained, by the supporters of this theory, 

 that its rapid general adoption showed that it supplied a 

 real want, as in point of fact it did. A want, however, may 

 be supplied in a wrong way, as was the case in this instance. 

 A void certainly did exist, but it is a marvel to me how it 

 could have been filled by the absurd phlogiston theory, 

 especially in the face of the masterly researches of Hooke 

 and Mayow, which, as I have already indicated, were 

 being conducted just about the time this theory was first 

 brought forward. The phlogiston theory, as most people are 

 aware, assumed the existence of a combustible principle, 

 which was called phlogiston, and which, I think, I cannot 

 better describe than by calling it the opposite of oxygen. 

 When a substance is burned, the supporters of this theory 

 taught that it lost phlogiston ; and when it was, so to speak, 

 unburned, they asserted that it gained phlogiston. Thus 

 mercury, if it is heated in contact with the air for a certain 

 time, loses its liquid form, and becomes a powdery solid. 

 This change, we are told by the phlogiston advocates, is 

 owing to the metal having lost phlogiston. If, again, the 

 reddish powder, obtained in the way just described, be heated 

 a little more strongly, another change takes place — the metal 

 reappears, owing, so says Stahl and his followers, to its 

 having absorbed phlogiston. That is to say, the metals, such 

 as mercury or lead, were regarded by Stahl as compounds, 

 whose constituents were phlogiston and the calx, or, as we 

 should say, the oxide of the metal. In other words, the loss 

 of phlogiston was identical with what we call absorption of 

 oxygen, and vice versd. Here, however, a difficulty arose. 

 It was noticed that substances, by losing phlogiston, became 

 heavier, and that by gaining it they became lighter. The 



