212 Biographical Memoir of Henry Cavendish. 



bodies which furnished them, although no one was able to point 

 out, with precision, in what these alleged emanations consisted. 

 Mr Cavendish presented his Memoir; and, in a few pages, 

 cleared up the subject. He compared, with each other, the 

 elastic fluids extracted from lime and alkalies, that produced by 

 fermentation and putrefaction, and that which occupies the bot- 

 toms of wells, caves, and mines ; and shewed that they have all 

 the same properties, and form but one and the same fluid, to 

 which the name oi fixed air was from that time restricted. He 

 determined the specific weight of this air, and found it always 

 the same, and greater by a third, than that of common air ; 

 which accounts for the low position it occupies, and the delete- 

 rious effects to which it gives rise in the bottom of cavities. He 

 discovered that this kind of air possesses the property of com- 

 bining with water, and then dissolving limestone and iron ; 

 which explains the effects of incrusting waters, the formation of 

 stalactites, and the presence of iron in mineral springs. Lastly, 

 he asserted, that it is precisely the same air that is developed 

 in the combustion of charcoal, and which renders that substance 

 so dangerous as an article of fuel. 



His experiments on inflammable air were still newer and more 

 striking. This fluid, which was only known by the explosions 

 sometimes produced by it in mines, had scarcely begun to 

 occupy the attention of philosophers at the time when he under- 

 took its investigation. Treating it in the same manner as the 

 former, he shewed that it is identical, and possesses the same 

 properties, whether it be obtained from the solution of iron, or 

 from that of zinc, or of copper ; and of these properties, he 

 more especially pointed out its specific lightness, which is about 

 ten times greater than that of common air ; and of which our 

 fellow member, M. Charles, afterwards made such a happy ap- 

 plication for rendering the navigation of the air by balloons 

 sure and easy. It may, in fact, be said, that without the dis- 

 covery of Mr Cavendish, and M. Charles's application of it, that 

 of Mr Montgolfier would scarcely have been practicable, so 

 many dangers and inconveniences did the fire, necessary for 

 keeping the air in his balloons expanded, occasion to the 

 aeronaut. 



But Mr Cavendish's investigation was followed by other re- 



