192 On the Chemical Constitution of 



tlie air had been withdrawn from the retort by exhaustion, alto- 

 gether preclude this supposition. But its utter inadequacy will, 

 to any one taking the trouble of repeating the experiment, be 

 sufficiently apparent. 



It is stated that, in the action of muriatic acid gas on metals, 

 hydrogen equal in bulk to half the volume of the gas is produced; 

 and therefore, it is added, if water had been generated by the 

 action of the acid gas on metals, it must have been the chlorine 

 or the metal, or both, that v/ere decomposed. " But in an ex- 

 periment of passing chlorine gas over ignited iron wire, not the 

 slightest appearance of moisture was perceptible." 



This argument, in common with some others of Sir H. Davy's 

 results, may apply with sufficient ^force to those experiments in 

 which it is said that water v^^as obtained equal, or nearly so, to 

 the whole quantity of water which, according to the old doctrine, 

 is contained in muriatic acid gas; for it is evident that this water 

 could not have been deposited and hydrogen also evolved. But 

 it does not apply to my experiments, in which a small though 

 very sensible portion of water was obtained ; for in such a case 

 hydrogen will also be produced, though not to the precise amount 

 of half the volume. The actual results, therefore, in the parti- 

 cular form of experiment employed, ought to have been ascer- 

 tained, instead of a general conclusion being reasoned on, more 

 especially when even the general fact is not so clearly established 

 that it can be held as demonstrated. The theoretical result no 

 doubt is, that hydrogen will be evolved equal to half the volume of 

 muriatic acid gas, siuce the latter is formed from equal volumes of 

 hydrogen and chlorine. But circumstances may occur connected 

 with the experiment, which will modify this. There is one ob- 

 vious circumstance of this nature, — that of the absorption of a 

 portion of the muriatic aciil gas by the muriate formed, — whence, 

 as the entire quantity of acid is not decomposed, the quantity of 

 hydrogen produced must, if the experiment be accurately per- 

 formed, appear less than half the volume. On this point accord- 

 ingly there has been considerable diversity of result. Sir H. Davy 

 himself at a former period, in experiments conducted with much 

 care and having no reference to theory, found that the quantity 

 of hydrogen evolved from the action of potassium, and of mer- 

 cury on muriatic acid gas, is equal only to about one third of the 

 original volume of the gas"^. When therefore the conclusion is 

 adopted as the ground of argument that the quantity is one half, 

 without any allusion to any difficulty in the experiment, any 

 source of fallacy attending it, or any opposite result having been 

 obtained, its inaccuracy, or at least its uncertainty, may be fairly 

 presumed. I had already observed in relation to this point, that 

 * Philosophical Transactions, ISOD-IO. 



