1908-9.] Life and Chemical Work of Archibald S. Couper. 205 
Both give us a much deeper view into Couper’s mode of thought. 
They show us how powerfully he was affected by his early philosophical 
training. In the introduction he developes in a general way the condi- 
tions which every sound theory must fulfil. The English paper begins 
with the two propositions : “ The end of chemistry is its theory. The 
guide in chemical research is a theory .” These he translates in the French 
version : — 
“ L’etude de la chimie doit avoir pour but l’etablissement de la theorie 
de cette science ; une theorie elle-meme est un guide qui nous conduit dans 
les recherches chimiques.” 
Couper then criticises the theories prevalent at that time, especially 
Gerhardt’s type theory as the most important, but also the radical theory 
and the theory of copulse. He rejects them all. He objects to the type 
theory that the peroxides do not fit into the types. Gerhardt’s types are,, 
as he himself says, types of double decomposition, but we cannot regard 
as double decomposition such actions as the formation of sulphuric acid 
from its anhydride and water. The multiple water types existed only in 
Gerhardt’s imagination. 
Couper now examines in how far Gerhardt’s type theory is in 
accordance with the fundamental requirements of philosophy, which 
demand that it should explain the greatest number of facts in the 
simplest possible way. In the next place, a test must be applied to see 
if it explains the facts at all ; and thirdly, if that is so, how it explains 
them. 
To these questions Couper answers that Gerhardt’s type theory 
does indeed, from a certain point of view, compare every chemical 
compound with every other, but that it does not explain the 
facts at all, so that obviously the how of the third question has no 
application. 
Couper’s sarcastic vein appears in a remarkable parallel which he 
draws in order to show the absurdity of the type theory : — “ Suppose that 
some one were to systematize the formation of letters into words that 
formed the contents of a book. Were he to begin by saying that he had 
discovered a certain word which would serve as a type, and from which 
by substitution and double decomposition all the others are to be derived , — 
that he, by this means, not only could form new words, but new books 
and books almost ad infinitum, — that this word also formed an admirable 
point of comparison with all the others, — that in all this there were only 
a few difficulties, but that these might be ingeniously overcome, — he would 
state certainly an empirical truth. At the same time, however, his method 
