426 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



substances to the type water, when one can effect similar 

 changes in them, and when the products are related in the 

 same way as those which result from the substitution of 

 other radicals for one of the radicals of water. For example, 

 I derive ether from the type water, because one can, by 

 double decomposition, replace the oxygen of ether by its 

 equivalent of chlorine, of bromine, of sulphur or of nitrogen, 

 to produce ethyl chloride, ethyl bromide, ethyl sulphide or 

 ethyl nitride (ethylamine) and because the products are 

 related in the same way as the hydrogen chloride, hydrogen 

 bromide, hydrogen sulphide and hydrogen nitride (ammonia), 

 which result from the substitution of the radicals chlorine, 

 bromine, sulphur and nitrogen, for the radical oxygen of 

 water" (Treatise, 1856, IV. 586). 



" To facilitate the classification of substances according 

 to their functions, one may, instead of taking water alone as 

 a formula-type, join to it, as derived types, compounds 

 which result from the displacement of the radical oxygen 

 from water, such as hydrogen chloride, hydrogen nitride, 

 etc. . . . The study of organic compounds, as will be seen 

 later, proves that the four types water, hydrochloric acid, 

 ammonia, hydrogen, suffice for a methodical classification. 

 These four formula-types may be noted as follows : 



Water OH 2 



Hydrochloric acid C1H 

 Ammonia NH 3 



Hydrogen HH 



equal volumes. 



" The water type includes the oxides (bases, acids, salts, 

 alcohols, etc.), the sulphides, the selenides and the 

 tellurides. 



" The hydrochloric acid type includes the chlorides, the 

 bromides, the iodides and the cyanides. 



" The ammonia type includes the nitrides and the 

 phosphides. 



" The hydrogen type includes the metallic hydrides and 

 the metals " (ibid. pp. 588-589). 



The following examples (with others) are given by 

 Gerhardt as illustrations of his theory of types : 



