Mr. A. S. Couper on o Neio Chemical Theory. 105 



the formulse of these bodies being, according to Gerhardt, in 

 their free state O.SO* and H^O. Combined, they become simply 

 SH* 04. 



The same remark applies in like manner to carbonic acid. In 

 these instances the wonted consequence of Gerhardt is missed. 

 The fact of the density of the vapour of these bodies being the 

 same in the free as in the combined states, may have prevented 

 him from doubling the formulae of these anhydrous acids. The 

 types of this theory being essentially types of double decomposi- 

 tion, this instance of simple combination diminishes somewhat 

 the value of the otherwise great logical merit of this system. 



Having taken notice of such excejitions, the empii-ical truth 

 of the theory may be otherwise admitted. 



The philosophical test demands that a theory be competent 

 to explain the greatest number of facts in the simplest possible 

 manner. 



In applying this test, three aspects of it require to be taken 

 into consideration : — 



1. As to the extension of the theory. 

 3. The explanation it affords of the facts. 

 3. The manner of this explanation. 



As to the first: this theory indeed brings every chemical com- 

 binate under a certain comparative point of view with every 

 other. Herein apparently is its merit. Nevertheless, should our 

 test be applied to its full extent, it will be found that it is fatal 

 to this system, in other respects so imposing. The comparative 

 point of view which it adopts is fundamentally false. 



As to the second : it does not explain the facts at all ; con- 

 sequently the most essential point of the test is unfulfilled. 



3. This condition of the test is in like manner unfulfilled, from 

 the fact of the second not being complied with. 



Why is it that Gerhardt's theory so signally fails in these two 

 essential requisites? Because it is based upon an old but vicious 

 principle, which has already retarded science for centuries. It 

 begins with a generalization, and from this generalization deduces 

 all the particular instances. But it does not come within the 

 limits of a chemical paper to enter upon a discussion which is 

 purely metaphysical. Nevertheless the theory of Gerhardt can 

 only be combated upon metaphysical grounds, because it is only 

 in overturning a general principle of research that the theory 

 can be proposed. Gerhardt's generalization lacks, moreover, the 

 merit of being represented by a type having a known existence. 



nO Ti, from which he derives every chemical combinatc, being in 



itself indefinite, cannot of course be contained or be produced 

 in any definite body. That, however, which may be demanded 



