This is owing to a very peculiar property of the vapour of ether, and indeed of many other combustible gaseous bodies. At a certain temperature lower than that of ignition, these vapours undergo a slow and imperfect combustion, which does not give rise, in any sensible degree, to the phenomena of light and flame, and yet extricates a quantity of caloric sufficient to react upon the wire and make it red-hot, and the wire in its turn keeps up the effect as long as the emission of vapour continues. CAROLINE. But why should not an iron or silver wire produce the same effect? MRS. B. Because either iron or silver, being much better conductors of heat than platina, the heat is carried off too fast by those metals to allow the accumulation of caloric necessary to produce the effect in question. Ether is so light that it evaporates at the common temperature of the atmosphere; it is therefore necessary to keep it confined by a well ground glass stopper. No degree of cold known has ever frozen it. CAROLINE. Is it not often taken medicinally? MRS. B. Yes; it is one of the most effectual antispasmodic medicines, and the quickness of its effects, as such, probably depends on its being instantly converted into vapour by the heat of the stomach, through the intervention of which it acts on the nervous system. But the frequent use of ether, like that of spirituous liquors, becomes prejudicial, and, if taken to excess, it produces effects similar to those of intoxication. We may now take our leave of the vinous fermentation, of which, I hope, you have acquired a clear idea; as well as of the several products that are derived from it. CAROLINE. Though this process appears, at first sight, so much complicated, it may, I think, be summed up in a few words, as it consists in the conversion of sugar and fermentable bodies into alcohol and carbonic acid, which give rise both to the formation of wine, and of all kinds of spirituous liquors. MRS. B. We shall now proceed to the _acetous fermentation_, which is thus called, because it converts wine into vinegar, by the formation of the acetous acid, which is the basis or radical of vinegar. CAROLINE. But is not the acidifying principle of the acetous acid the same as that of all other acids, oxygen? MRS. B. Certainly; and on that account the contact of air is essential to this fermentation, as it affords the necessary supply of oxygen. Vinegar, in order to obtain pure acetous acid from it, must be distilled and rectified by certain processes. EMILY. But pray, Mrs. B., is not the acetous acid frequently formed without this fermentation taking place? Is it not, for instance, contained in acid fruits, and in every substance that becomes sour? MRS. B.