386 Dr. Hofmann on the Metamorphoses of Indigo. 



foretell these changes a complete knowledge of the constitution 

 of the original body was necessary, and that this must be in- 

 cluded in the argument. This discussion is not yet terminated, 

 nor are we at present in a position to give an opinion as to 

 its final result; this however is certain, that it has been the 

 means of originating many excellent investigations, from which 

 science has derived the greatest advantage. 



It is now eight years since Laurent was led by a series of 

 experiments to a peculiar mode of representing the action of 

 chlorine on organic bodies, and among other views, of which we 

 do not require here to speak, he laid down the following law: — 



" In many organic compounds, by the action of chlorine, 

 a certain number of equivalents of hydrogen are eliminated in 

 the form of hydrochloric acid ; the eliminated hydrogen being 

 replaced by a like number of equivalents of chlorine, in such 

 a manner, that the chemical character and the fundamental 

 properties of the original compound are not sensibly altered. 

 The chlorine molecules therefore must take the place of the 

 hydrogen molecules, and play in the new compound the same 

 part which was performed by the hydrogen molecules in the 

 original." 



The ideas of Laurent, which were opposed to the electro- 

 chemical theory, and to the views generally received at that 

 time, found, as might be expected, but an indifferent recep- 

 tion from chemists at the period of their publication. His 

 views were reproached with being supported by too small a 

 number of facts : even his facts were assailed, and the author 

 required all his dexterity and steady perseverance to defend 

 himself against his powerful opponents. He repeated many 

 of his former experiments, and added by further investiga- 

 tions new proofs to those already given. Since that time a 

 mass of facts has been observed which unequivocally sup- 

 port the ideas of Laurent. The discovery of chloracetic acid 

 by Dumas, the investigations of Regnault on the action of 

 chlorine on the Liqueur des Hollandais, of Malaguti on the 

 action of the same body on the compound aethers, the ex- 

 tended works of Laurent on the naphthaline and phenyle 

 series, the researches of the same chemist and of Erdmann 

 on indigo, have given so many instances in which any other 

 view is inadmissible, that it would appear scarcely necessary 

 to undertake still further investigations in its support. 



On examining however more closely the bodies in wiilch 

 hydrogen has hitherto been replaced by chlorine in the way 

 mentioned, it is found that they consist of indifferent bodies, 

 or acids, or their compounds with aether. Up to the present 

 time the theory of substitution remains unrepresented by a 



