210 



ON THE Jm. AND .KAIN OF MANCHESTER. 



rity in it ; to some it means a different kind of air inherently 

 impure. This vague and alchemistic notion, which did not 

 distinctly define, was combatted in speaking of compounds 

 generally, even by Roger Bacon, but it was not until Dalton's 

 time that any distinct notions of compound bodies became 

 general, notwithstanding Newton's definition of atoms. 



The number of analyses of air in order to ascertain its 

 oxygen and nitrogen, have been very great, and many also 

 exist which determine the carbonic acid ; little, however, has 

 been done in ascertaining the other possible contents, and 

 some eminent names have rather discouraged the idea of find- 

 ing any thing, because if any thing else did exist, the amount 

 would be too small for analysis to detect, and therefore too 

 small for the body to be affected by it. Some such mode of 

 reasoning has been current, but it is not in our power to tell 

 what is the smallest amount of matter which will affect the 

 system, and we know that exceedingly small doses affect us 

 if taken repeatedly, although that effect is not equal to the 

 effect of taking the whole at once. 



Whatever chemistry or scientific men have said about the 

 air, or indeed about any thing else, common sense and ordi- 

 nary observation have had their force in no way diminished, 

 and it is wiser for science to explain the large generalizations 

 of common sense and the teachings of instinct, than to run 

 counter to them ; by which means it produces on one side at 

 times extraordinary scepticism, and on the other extraordinary 

 credulity. 



Our senses are often much superior as tests to chemical tests, 

 that is, we can perceive by our senses very often less than we 

 can test; but it is not always the case, and when the amount 

 is too small for either mode of observation, experiment has 

 the great advantage of being able to condense and to accumu- 

 late, and so bring everything within the region of the sight. 

 But it is not to be supposed that because we bring within the 

 limits of a glass vessel enough of the impurities of the atmo- 



