On substituting Iodine for Hydrogen in Organic Compounds. 201 



By the foregoing investigations, therefore, the law that all 

 bodies whose conducting power is increased by heating are elec- 

 trolytes, is now freed from those exceptions which it appeared to 

 suffer j neither is it necessary, although many may yet feel in- 

 clined to do so, to ascribe the apparently incomplete electrolysis 

 to a double conduction, as Foucault has lately assumed, and as 

 Earaday himself has already acknowledged. Even if the hypo- 

 thesis of such a coexistence of two conductions involves nothing 

 contradictory in itself, it is corroborated by no single experiment. 

 The proofs submitted by Foucault in favour of his views rhay be 

 considered as disproved by the opposite experiments of Buff*. 

 The experiments communicated by Foucault f at a later period 

 are based upon an erroneous conception of the term branch- 

 current; they can possess no weight whatever, inasmuch as 

 formulae dependent upon this erroneous conception have been 

 improperly introduced into the observations. At any rate the 

 above experiments show that, even in the weakest conduction by 

 an electrolyte, the latter is electrolytic ; and hence that the co- 

 existence of both kinds of conduction can never take place in the 

 manner of branch-conductions, but at most that in the electrolysis 

 every moving atom again conducts. 



XXVIII. On a General Method of Substituting Iodine for 

 Hydrogen in Organic Compounds^ and on the Properties of 

 lodopyromeconic Acid. By Mr. James F. Brown, Assistant 

 to Professor Anderson, Glasgoiv%* 



IN a paper on pyromeconic acid read before this Society, 

 and since published in the Philosophical Magazine for 

 September 1852, I have detailed the preparation and properties 

 of a compound obtained by the substitution of an equivalent 

 of bromine for an equivalent of hydrogen in that acid. Having 

 observed that this substitution was very easily effected, 1 was 

 induced to attempt the formation of an lodopyromeconic acid, in 

 the hope of adding one to the very few instances in which the 

 direct substitution of iodine for hydrogen has been found pos- 

 sible. For this purpose I digested pyromeconic acid with tinc- 

 ture of iodine, but no success attended the experiment, the acid 

 remaining entirely unchanged. The failure of this attempt led 



from the proper values which are caused by introducing those slightly erro- 

 neous numbers into calculation. I may also mention, that early in April 

 I had already communicated my results to many of my colleagues in 

 Berlin. (To which 1, for my part, gladly bear testimony. — Poggendorff.) 



* Annalen der Chemie und Pharm. vol. Ixxxviii. p. 177- 



t Cosmos, vol. iv. p. 248. 



X From the Transactions of the Royal Society of Edinburgh, vol. xxi. 

 part 1 ; read April 3, 1854. vrnvt; *''<< < 



