other points, I endeavoured to devise an arrangement by which I could decompose a body against a surface of water, as well as against air or metal, and succeeded in doing so unexceptionably in the following manner. As the experiment for very natural reasons requires many precautions to be successful, and will be referred to hereafter in illustration of the views I shall venture to give, I must describe it minutely.
230. A glass basin (fig. 12), four inches in diameter and four inches deep, had a division of mica a, fixed across the upper part so as to descend one inch and a half below the edge, and
Fig. 12.
be perfectly water-tight at the sides: a plate of platina b, three inches wide, was put into the basin on one side of the division a, and retained there by a glass block below, so that any gas produced by it in a future stage of the experiment should not ascend beyond the mica, and cause currents in the liquid on that side. A strong solution of sulphate of magnesia was carefully poured without splashing into the basin, until it rose a little above the lower edge of the mica division a, great care being taken that the glass or mica on the unoccupied or c side of the division in the figure should not be moistened by agitation of the solution above the level to which it rose. A thin piece of clean cork, well wetted in distilled water, was then carefully and lightly placed on the solution at the c side, and distilled water poured gently on to it until a stratum the eighth of an inch in thickness appeared over the sulphate of magnesia; all was then left for a few minutes, that any solution adhering to the cork might sink away from it, or be removed by the water on which it now floated; and then more distilled water was added in a similar manner, until it reached nearly to the top of the glass. In this way solution of the sulphate occupied the lower part of the glass, and also the upper on the right-hand side of the mica; but on the left-hand side of the division a stratum of water from c to d, one inch and a half in depth, reposed upon it, the two presenting, when looked through horizontally, a comparatively definite plane of contact. A second platina pole e was arranged so as to be just under the surface of the water, in a position nearly horizontal, a little
