ments have been frequently repeated with the same results; and if it be proved by further investigation, that chemical changes in worts are retarded by negative surfaces, coolers may be constructed on this principle, and reservoirs for preserving beer, in which a second fermentation or fret (as it is technically termed) will be less apt to occur: an accident, which is so often a cause of great loss to practical brewers. Vessels composed of zinc and copper are, of course, objectionable on account of their poisonous qualities. We find, however, from the investigations of M. Martens, member of the Royal Academy at Brussels, that iron may be rendered passive or negative, by a dull red heat, insomuch that it is not affected by the unfuming nitric acid of commerce, which powerfully affects common iron. The immersion of iron in highly concentrated nitric acid has the same effect, and so has also the crystallisable, or highly concentrated acetic acid. Passive vessels may, consequently, be made of this metal which is, I believe, already used for these purposes, without this important preparation.
It is not our intention to enter into any theoretical consideration of these phenomena; we merely point them out to practical men, in order to induce them to avail themselves of the opportunities they enjoy, in conducting their great processes, by their observations and experiments, to aid us in tracing
