In the second case, that of silver, we have its direct oxidation by the free oxygen present, and the reaction of these oxides thus formed upon the salt in its vicinity, argentic chloride and caustic potash resulting, to which last compound, of course, that alkalinity is induced which we observe.

In the case of zinc it may be that the reactions which result in this alkalinity of the saline liquid surrounding it may not be so simple as I here suppose, further investigations seem indeed requisite ere we can fully explain them; but still the results I have described, and several others I could cite of an analagous nature to these, certainly tend to show that the conception of metallic “replacement,” as given in this paper of Professor Gladstone, is as yet scarcely a tenable one, or at least that it requires for adequate support considerably more evidence than has yet been tendered in its behalf.

With regard now to the next statement I have to remark upon, viz., that “Mercury and gold in conjunction would decompose mercuric chloride with deposition not only of lower chloride, but also of metallic mercury,” I think it very possible that floating dust or other impurities, or even light had in some way interfered with what should be the legitimate result of the experiment described. In support of this view I found that mercury, which, for greatest purity, I electro-deposited from its potassic-cyanide upon platinum, when not in presence of mercuric-chloride, and kept in the dark away from dust, gave a deposit upon gold of mercurous-chloride only. I may state that the detection of either mercurous-chloride or mercury here appears to me much facilitated if platinum be used in place of gold for the receiving plate, as this metal has its lustre greatly dimmed by even traces of adherent chloride, and any mercury present is easily rubbed off upon an angle of gold, and thus readily identified. Using this modification of Professor Gladstone's apparatus, I was only able to get, even in sixteen hours, a deposit of mercurous-chloride of sufficient thickness to perceptibly impair the lustre of the platinum upon which it formed; its presence, in fact, could not be certainly detected except by the slight darkening of this platinum in caustic potash. By the addition of pure hydrochloric acid, however, to the mercuric-chloride, thicker deposits of this kind were obtained, but none of mercury.

The deposit of this mercurous salt, however, even alone, under the circumstances as found and described by these investigators, appears to me a very suggestive phenomenon, because, upon first sight, it appears inexplicable. I can only attribute the formation of this deposit to the action of a free acid or acids upon the mercury; a minute quantity of nitric or nitrous compounds dissolved in the saline solution used (taken from the air, etc.) would be competent to thus act upon mercury to the extent