6 Prof. Johnston on the Iodide of a new Carbo-hydrogen. 



to the presence of much muriatic acid in the solution, derived 

 from the action of the chlorine on the alcohol, that this ra- 

 dical fg R unites with H CI (2°) instead of simply with CI, 

 when it might have been represented by a formula * the exact 

 counterpart of that which indicates the substance from which 

 it is derived. 



There is, however, still another mode of representing the 

 rational constitution of this substance, which while it is ac- 

 cordant with the facts on which Dumas's views are founded, 

 is inconsistent with the principle of the conservation of types. 

 The yellow compound may be 



(C 15 H 8 2 + HO) + (C 15 H 8 CI 2 ) . . . (4.) 



consisting of equal atoms of an analogous oxide and chloride 

 of the radical C 15 H 8 or C 30 H 16 , with the former of which is 

 combined also an atom of water. This mode of representing 

 it is accordant with the views of Berzelius, and is supported 

 by many interesting and striking analogies. 



We ought indeed to distinguish carefully between the fact 

 of the mutual substitution of hydrogen and chlorine, and the 

 theory of the persistence of types, or the opinion that the 

 element which replaces performs the same function in the 

 organic compound as that which is replaced. Of the former 

 there can be no doubt, while the adoption of the latter as a 

 principle is as yet attended with many difficulties and appa- 

 rent anomalies, which do not present themselves when we 

 regard these altered compounds after the manner in which 

 our yellow substance is represented in formula 4-. 



New views all tend to hasten forward science, but a new view 

 is not in itself necessarily an advance. It may often serveas a 

 useful guide-post, when it does not directly help us on our 

 way. Such good results are sure to follow from the discus- 

 sion of the theory of substitutions, though all the views of its 

 eminent author should not find a permanent place in the 

 science. 



I speak with the less confidence in regard to the above 

 formulae, because I am sensible that the examination of the 

 two compounds described in this paper is by no means com- 

 plete: a more careful research into the properties and che- 

 mical relations, especially of the first of them, would be likely 

 1 think to lead to interesting results. 



The analyses above given were made as far back as 1838, 

 and in February 1839, and the investigation was left unfinished 

 till I should obtain a fresh supply of the compound. My 



*C 30 H ^O 3 +Cl. 



