2l6 HISTORY OF CHEMISTRY. [LECTURE XI. 



but he connects them by means of transition compounds, and 

 thus obtains series in which he arranges substances. In arrang- 

 ing these series he makes use of two generalisations, of which, 

 however, one does not originate with himself. In 1842, 

 Schiel had pointed out 61 that the alcohol radicals form a series 

 whose separate members differ by n. CH 2 , and that the corre- 

 sponding alcohols show a difference in boiling point of 18 for 

 each CH 2 as H. Kopp had already proved in the case of ethyl 

 and methyl compounds. 62 In 1843, Dumas showed 63 that the 

 fatty acids also possess, amongst themselves, the same differ- 

 ence in composition. Gerhardt now employs this very striking 

 regularity, which, as is well known, occurs amongst very many 

 organic substances, and calls the compounds which differ by 

 n. CH 2 , homologous. It had been found that such compounds 

 possess great similarities to one another, and that their 

 physical properties slowly and progressively change. This 

 had especially appeared from Kopp's detailed and excellent 

 investigations, 64 Gerhardt establishes, further, the idea of 

 isologous compounds : these substances are also chemically 

 similar but their difference in composition is not n. CH 2 . 

 Acetic and benzoic acids are well-known examples belonging 

 to this class of substances. 



The homologous and isologous series constitute the one 

 part of Gerhardt's classification ; the other part is represented 

 by the heterologous series. All substances are referred to the 

 latter which can be obtained from one another by means of 

 simple reactions (by double exchange) ; these substances are 

 allied in their mode of formation but they are chemically 

 different. Gerhardt very appropriately regards this arrange- 

 ment of the compounds as similar to a game of cards which is 

 based upon the colour as well as upon the value of the separate 

 cards. Just as in the latter every card which is wanting is 

 characterised. by its vacant place as of a certain value and of a 

 certain colour, so the chief properties, the formation, and the 



61 Annalen. 43, 107. 62 Ibid. 41, 79. & Ibid. 45, 330. 64 Ibid. 

 41, 169 ; 50, 71 ; 55, 166 ; 64, 212 ; 92, I ; 94, 257 ; 95, 121, 307 ; 96, 



i, i53> 303 ; 98, 367 ; ioo, 19, etc. 



