474 NOTES. 



differentes." He then mentions Von Helm ont inaccurately- 

 as having ascertained that there were " permanently elastic 

 vapours other than atmospheric air ; " and Hales, still more 

 inaccurately, as having measured these permanently elastic 

 fluids : whereas Hales considered thera all as common air. 

 combined with impure exhalations, an opinion which pre- 

 vailed a century and a half after Yon Helmont, and was 

 adopted hy D'Alembert in his article "Air" in the Ency- 

 clopedic, 1751. Black's discovery of fixed air, he confines 

 solely to its explaining the causticity of the alkalis and 

 earths. No one, he says, before Cavendish had distinguished 

 it as a separate aeriform substance ; and though emanations 

 were said to proceed from bodies, no one knew in what they 

 consisted. Cavendish, he says, first settled in 1766 all these 

 questions, and showed that this air, whether from chalk or 

 fermentation, or mines, was one and the same fluid, " auquel 

 on a depuis reserve le nom d'air fixe." Finally he (Caven- 

 dish) discovered that it was the air evolved from burning 

 charcoal (p. cxxx). He then ascribes the application of in- 

 flammable air to raising balloons in the air to M. Charles's 

 application of Mr. Cavendish's experiments on the specific 

 gravity of that gas. 



This is really somewhat astounding. That a person of 

 M. Cuvier's eminent attainments, filling the high office of 

 Secretaire Perpetuel, and charged with the delicate and im- 

 portant duty of recording the history of science yearly, 

 should not have deemed it worth his while to read either the 

 celebrated experiments on Magnesia Alba and Quicklime 

 published in 1755, or the Lectures published in 1803, before 

 assuming to write the history of chemical discovery, is 

 wholly beyond belief. Had he read the former work he 

 would have found that Dr. Black gave to the air which he 

 had discovered the name of fixed air ; and that he did so, 

 not because it was the same with, or any modification of, 

 atmospheric air, but simply because air was a known term 

 in common use to represent a permanently elastic fluid, and 

 because this kind of air was found fixed in combination with 

 bodies. Had he looked at the Lectures, he would have 

 found that two years after the publication of his capital dis- 

 covery, viz., in 1757, and nine years before Mr. Cavendish's 

 paper was received, Dr. Black discovered that fixed air is 



