1614.] Daltonian Theory of Chemical Proportions. li 



Article II. 



On the Daltonian Theory of Chemical Proportions. 

 By Thomas Thomson, M.D. F. R.S. 



(Continued frtm vol. iii. p. 378.) 



I have hitherto avoided noticing muriatic acid and chlorine on 

 account of the difference of opinion which still exists respecting 

 the nature and composition of these hodies. But as muriatic acid 

 is more used by chemists than almost any other re-agent, the 

 knowledge of the constituents of the compounds which it forms is 

 of the utmost consequence. On that account I have been induced 

 to wave that delicacy which the handling of a controversial subject 

 requires, and to sacrifice etiquet to utility. 



As all attempts to decompose chlorine, and to obtain oxygen gas 

 from it, which have been made by Gay-Lussac and Thenard, Davy, 

 and various other persons, have entirely failed, I do not see how it 

 is possible to avoid considering it, in the present state of our know- 

 ledge, as a simple substance. If we neglect this rule, namely, 

 to deduce all our principles in chemistry from experiment, and 

 never to proceed farther than we are warranted by experiment, we 

 cannot possibly avoid going astray, and filling the science with 

 hypotheses and absurdities. I think it very likely, from the great 

 weight of the atoms of the metals, of chlorine and iodine, that 

 they are all compounds ; but we cannot introduce any such opinion 

 into the science, far less make it the foundation of our reasoning 

 and deductions, till some evidence be brought forward from expe- 

 riment that our opinion is well founded. Analogical reasoning is 

 always dangerous ; and in no science is it so apt to mislead as in 

 chemistry. Lavoisier's hypotheses respecting the acidifying prin- 

 ciple, and the composition of the alkalies, furnish very striking 

 proofs of the truth of this proposition. 



1 have carefully considered the arguments advanced by Mr. 

 Murray, Professor Berzelius, Mr. Henderson, and the French 

 chemists, in support of the opinion that chlorine is a compound of 

 muriatic acid and oxygen ; but I do not think that they have been 

 able even to establish an analogy in favour of their opinion. Mr. 

 Murray's papers are all written with a clearness, force, and acute- 

 new, that does him gieat credit. Indeed, nothing can aftbrd a 

 more striking proof of his abilities than the plausible point of view 

 in which he has contrived to place an hypothesis in favour of which 

 not a single decisive experiment, or even analogy, can be adduced. 

 In some particulars he has even the advantage in the argument ; an 

 advantage which it would have been easy to have anticipated from 

 Sir Humphry Davy's turn of mind: for the merit of this illustrious 

 chemist by no means consists in the minute precision of his experi- 



