26 Improvements in Physical Science (Jan. 
to three; namely oxygen, iodine, and chlorine; and, if Ampere’s 
hypothesis respecting fluorine, so ably supported by Sir Humphry 
Davy, be correct, it will constitute a fourth. Many important 
facts respecting these bodies have been lately ascertained. I shall 
state the principal of them in this place. 
1. Owygen. This substance was raised by Lavoisier to a very high 
rank among chemical substances. He considered it as the acidify- 
ing principle, as the only supporter of combustion, and as capable 
of uniting with and modifying all other simple bodies. The 
modern discoveries in chemistry have deprived oxygen of a good 
deal of its dignity. Davy has shown that it forms alkalies as well 
as acids, and that many acids exist which contain no oxygen. It is 
not therefore the acidifying principle. ‘This indeed is a doctrine 
which was all along maintained by Berthollet, whose sagacity in 
many points of chemical theory deserves the highest admiration. 
Oxygen has lost likewise the property of being the only simple 
supporter of combustion. For chlorine possesses that property per- 
haps in a greater degree than oxygen; with this curious exception, 
that charcoal will not burn in it nor unite with it. Iodine is ‘cer- 
tainly a much less perfect supporter of combustion, since the only 
body hitherto observed to burn in it is potassium. It is amusing to 
observe the awkward attempts of the French chemists to preserve for 
oxygen the exclusive privilege of being the only simple supporter of 
combustion. According to them combustion in the chemical sense 
of the word is very different from the meaning which it bears 
among the vulgar. Nothing, says Thierry, is more similar to 
combustion than what takes place when phosphorus is introduced 
into chlorine gas. We have flame and the phosphorus disappears. 
Nothing, on the other hand, is more unlike combustion, than the 
rusting of iron in a damp place. Yet the first, he informs us, is 
not a real combustion, while the second is. (Annales de Chimie, 
xciii. p. 53.) It is surprising that these gentlemen do not 
perceive that they are merely altering the meaning of a word, which 
has been known and understood ever since mankind were acquainted 
with fire. The burning of phosphorus in chlorine would be called 
comlustion by every person of common sense who witnessed the 
phenomenon. Nor is there any thing in the chemical mean- 
ing of the term, which is incompatible with its application to this 
and many other similar cases. The rusting of iron in a damp 
place would never be called combustion either by the vulgar or by 
chemists, who considered the case with attention. It consists 
merely in the transfer of the oxygen of water to the iron, Thenard 
and Gay Lussac have arranged chlorine and iodine among com- 
bustible substances, merely because they have the property of 
combining with oxygen. If they had placed these bodies in a class 
by themselves, their conduct might have been exeusable; but to 
call them combustible is absurd; because nothing similar to com- 
bustion, in any sense of the word, takes place when they unite 
with oxygen. The union cannot be directly accomplished, and_is 
A 
