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Proceedings of the Royal Society of Edinburgh. [Sess. 
APPENDIX Ilia. 
On a New Chemical Theory. By Archibald S. Couper, Esq. 
(Communicated by the Author.*) 
The end of chemistry is its theory. The guide in chemical research is a theory. 
It is therefore of the greatest importance to ascertain whether the theories at 
present adopted by chemists are adequate to the explanation of chemical phenomena, 
or are, at least, based upon the true principles which ought to regulate scientific 
research. 
Among those which have lately been developed, there is one, on account of its 
apparently numerous merits, which particularly claims investigation, and respecting 
which we deem that it would not be unprofitable were either new proofs of its 
scientific value furnished, or, on the contrary, should considerations be adduced 
establishing not only its inadequacy to the explanation, but its ultimate detriment 
to the progress of science. I allude to the system of types as advocated by 
Gerhardt. 
This system, striking alike for the breadth of its conception, and the logical and 
consequent manner in which it has been developed, has been controverted from the 
point of view afforded by theories less far-reaching than the one under consideration, 
and even based upon a one-sided and restricted appreciation of certain chemical 
reactions. The consequence is that this opposition has not impaired the favour with 
which the unitary system has been received, but has rather tended to display it in a 
more advantageous light. 
Imposing as this theory is, it is nevertheless all the more necessary to submit it to 
a strict investigation ; for there is nothing so prejudicial in the search for truth as 
the blind spirit of conservation. A rational belief demands the test of a preliminary 
doubt. 
There are two conditions which every sound theory must fulfil : — ■ 
1. It must be proved to be empirically true. 
2. It must no less be philosophically true. 
I admit that this theory is for the most part empirically true ; that is to say, it is 
not contradicted by many of the facts of the science. Evidence that this condition 
is only partially fulfilled, is to be found — 
1. In the circumstance that the peroxides, for instance, do not fit very satis- 
factorily into the types. 
2. The principle of double decomposition cannot ivell be applied to the con- 
version of the anhydrous sulphuric acid into the hydrate of that acid by the 
action of one equivalent of water, the formulae of these bodies being, according 
to Gerhardt, in their free state O.SO 2 and H 2 0. Combined, they become simply 
SH 2 0 4 . 
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 decomposition , this instance of a simple 
* Phil. Mag ., vol. xvi. , 4th series (1358), pp. 104-116. 
