XX.] THEORY OF TYPES IN CHEMISTRY. 461 



1855, represent different degrees of condensation of matter. 

 The molecule of hydrogen, H 2 = (M 2 ), corresponding to four 

 volumes, combines with two volumes of oxygen (0 2 ) to form, 

 four volumes of water, and may thus be regarded as condensed 

 to one half in its union with oxygen, and derived from a 

 double molecule, M 2 M 2 . In like manner four volumes of 

 ammonia contain two volumes of nitrogen and six of hydro- 

 gen, which, being reduced to one third, correspond to a triple 

 molecule, M 3 M 8 , so that these three types and their multiples 

 are reducible to that of hydrogen more or less condensed. 

 (Wurtz, Annales de Chimie et de Physique (3), XLIY. 304.) 



As regards the rejection of water as a type of organic com- 

 pounds, and the substitution of carbonic acid, founded upon 

 the consideration that these in nature are derived from C 2 4 , 

 Wurtz has well remarked that water, as the source of hydro- 

 gen, is equally essential to their formation, and, indeed, that 

 the carbonic anhydride C 2 04, 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 anhydride, first, because we can com- 

 pare with it many mineral compounds which can with diffi- 

 culty 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 car- 

 bonic 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, mole- 

 cules 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 ex- 

 pounders of this doctrine of molecular types as above defined : 

 now almost universally adopted by chemists. He writes, 

 " To my mind this idea of referring to water, taken as a type, 

 a very great number of compounds, is one of the most beauti- 



