122 ON THE THEORY OF TYPES IN CHEMISTRY. 
volumes of nitrogen and six of hydrogen, which, being reduced to 
one-third, correspond to a triple molecule, M,M,, so that these 
three types and their multiples are reducible to that of hydrogen 
more or less condensed.—(WuR1Zz, Annales de Chimie et de Physique. 
(3) xliv. 304). 
As regards the rejection of water as a type of organic compounds, 
and the substitution of carbonic acid, founded upon the considera- 
tion that these in nature are derived from ©,0,, Wurtz has well 
remarked that water, as the source of hydrogen, is equally essential to 
their formation, and indeed that the carbonic anhydrid C,0O,, like all 
other anhydrous acids, may be regarded as a simple derivative of the 
water type. Having then adopted the notion of referring a great 
variety of bodies to a mineral species of simple constitution, water 
is to be preferred to carbonic anhydrid, first, because we can com- 
pare with it many mineral compounds which can with difficulty be 
compared with carbonic acid; and secondly, because the two atoms 
of water being replaceable singly, the mode of derivation of a great 
number of compounds (acids, alcohols, ethers, etc.,) is much more 
simple and natural than from carbonic acid. As Wurtz happily 
remarks, Kolbe has so fully adopted the theory of types that he 
‘wishes to multiply them, and even admits condensed types, which 
are, however, molecules of carbonic acid and not of water; “he 
combats the types of Gerhardt and at the same time counterfeits 
them.” 
Thus far we are in accordance with Mr. Wurtz, who has shown 
himself one of the ablest and most intelligent expounders of this 
doctrine of molecular types, as above defined, now almost univer- 
sally adopted by chemists. He writes,—“ to my mind this idea of 
referring to water, taken as a type,a very great number of com- 
pounds, is one of the most beautiful conceptions of modern chemis- 
try.’—(Repertoire de Chimie Pure, 1860, p. 859); and again, he 
declares the idea of regarding both water and ammonia as represen- 
tatives of the hydrogen type, more or iess condensed, to be so simple 
and so general in its application that it is worthy “to form the basis 
of a system of chemistry.—(Ibid. p. 356.) 
We have in this theory two important conceptions : the first is that 
of hydrogen and water regarded as types to which both mineral and 
organic compounds may be referred; and the second is the notion 
of condensed and derived types, according to which we not only. 
