CONDITIONS OP THE CONSTITUTION OF ELECTROLYTES. 81 



675.. Amongst powerful compounds which are not decomposed, boracic acid stands 

 prominent (408.). Then again, the iodide of sulphur, and the chlorides of sulphur, 

 phosphorus, and carbon, are not decomposable under common circumstances, though 

 their elements are of a nature which would lead to a contrary expectation. Chloride 

 of antimony (402. 690.), the hydro- carbons, acetic acid, ammonia, and many other 

 bodies undecomposable by the voltaic pile, would seem to be formed by an affinity suf- 

 ficiently strong to indicate that the elements were so far contrasted in their nature 

 as to sanction the expectation that the pile would separate them, especially as in some 

 cases of mere solution (530. 544.), where the affinity must by comparison be very weak, 

 separation takes place*. 



676. It must not be forgotten, howevei-, that much of this difficulty, and per- 

 haps the whole, may depend upon the absence of conducting power, which, pre- 

 venting the transmission of the current, prevents of course the effects due to it. All 

 known compounds being non-conductors when solid, but conductors when liquid, are 

 decomposed, with perhaps the single exception at present known of periodide of mer- 

 cury (679. 691.) ; and even water itself, which so easily yields up its elements when 

 the current passes, if rendered quite pure, scarcely suffers change, because it then 

 becomes a very bad conductor. 



677' If it should hereafter be proved that the want of decomposition in those 

 cases where, from chemical considerations, it might be so strongly expected (669. 674. 

 672.), is due to the absence or deficiency of conducting power, it would also be proved, 

 at the same time, that decomposition depends upon conduction, and not the latter upon 

 the former (413.) ; and in water this seems to be very nearly decided. On the other 

 hand, the conclusion is almost irresistible, that in electrolytes the power of transmit- 

 ting the electricity across the substance is dependent upon their capability of suffering 

 decomposition ; taking place only whilst they are decomposing, and being propor- 

 tionate to the quantity of elements separated (821.). I may not, however, stop to 

 discuss this point experimentally at present. 



678. When a compound contains such elements as are known to pass towards the 

 opposite extremities of the voltaic pile, still the proportions in which they are pre- 

 sent appear to be intimately connected with capability in the compound of suffering 

 or resisting decomposition. Thus, the protochloride of tin readily conducts, and is 

 decomposed (402.), but the perchloride neither conducts nor is decomposed (406.). 

 The protiodide of tin is decomposed when fluid (402.); the periodide is not (405.). 

 The periodide of mercury when fused is not decomposed (691.), even though it does 

 conduct. I was unable to contrast it with the protiodide, the latter being converted 

 into mercury and periodide by heat. 



679. These important differences induced me to look more closely to certain binary 

 compounds, with a view of ascertaining whether a laiv regulating the decomposahility 



* With regard to solution, I have met with some reasons for supposing that it will probably disappear as a 

 cause of transference, and intend resuming the consideration at a convenient opportunity. 

 MDCCCXXXIV. M 



