heat at one of the junctions, only thermo-currents can be produced. Some peculiar effects of heat are noticed and explained ; and several very necessary precautions in conducting these experiments are pointed out ; and it is found, when these are taken, that heat has a decided and distinct effect over the chemical affinities of the parts of a circuit subjected to its power, and a corresponding influence on the electric current produced. This proceeds to such an extent, that, in some cases, either of two metals can be made positive or negative with respect to the other in the same fluid, solely by virtue of this power of heat.
The effect o^ dilution is then examined. For this purpose, only one metal and one fluid are used in a circuit ; but the fluid is rendered more dilute at one point of contact than at the other. It was ascertained that such dilution produces little or no effect with metals which are not acted on by the electrolyte employed ; and the precautions requisite as to other points are then stated. But when these are observed, still dilution is found to have a most powerful influence on the results ; and, as the author believes, solely on account of its influence on the active chemical affinity. Thus copper in dilute nitric acid is positive with respect to copper in strong nitric acid ; and the same is the case with lead, silver, and other metals. It is not that the piece in the weakest acid is always positive with respect to that in the stronger acid ; for, in the first place, some very curious cases are given, in which a piece of metal in acid of a certain strength is positive with respect to a piece of the same metal in acid, either stronger or weaker ; and, in the next place, other cases are stated in which the piece in the medium acid is negative with respect to the other piece in either stronger or weaker acid. The effect of dilution in nitric acid is such, that when certain different metals are compared together, either can, at plea- sure, be made positive or negative with respect to the other ; thus, of the five metals, silver, copper, iron, lead, and tin, any one of them can be made either positive or negative with respect to any other ; with the sole exception of silver, which is always positive with re- spect to copper. The inconsistency of these results with any theory of contact electromotive force is then strongly insisted on by the author.
The next division of the paper treats of the order of the metallic elements of voltaic circuits when different electrolytes are used. It is usual to say, that metals are positive or negative with respect to each other in a certain order ; but Davy, and afterwards De la Rive, showed that, in certain cases, this order is inverted. The author, by using ten metals, and seven different exciting electrolytic solu- tions, shows that in no two solutions is the order the same ; but that changes of the most extreme kind occur in exact conformity with the changes in chemical action, which the use of the different solutions occasions.
The next division of the paper considers the very numerous cases in which voltaic circuits, often such as are able to effect decomposition, are produced without any metallic contact, and by virtue of
