I know that in painting, the lakes are reckoned far less durable colours than the ochres; but what is the reason of it? MRS. B. The change which takes place in vegetable colours is owing chiefly to the oxygen of the atmosphere slowly burning their hydrogen, and leaving, in some measure, the blackness of the carbon exposed. Such change cannot take place in ochre, which is altogether a mineral substance. Vegetable colours have a stronger affinity for animal than for vegetable substances, and this is supposed to be owing to a small quantity of nitrogen which they contain. Thus, silk and worsted will take a much finer vegetable dye than linen and cotton. CAROLINE. Dying, then, is quite a chemical process? MRS. B. Undoubtedly. The condition required to form a good dye is, that the colouring matter should be precipitated, or fixed, on the substance to be dyed, and should form a compound not soluble in the liquids to which it will probably be exposed. Thus, for instance, printed or dyed linens or cottons must be able to resist the action of soap and water, to which they must necessarily be subject in washing; and woollens and silks should withstand the action of grease and acids, to which they may accidentally be exposed. CAROLINE. But if linen and cotton have not a sufficient affinity for colouring matter, how are they made to resist the action of washing, which they always do when they are well printed? MRS. B. When the substance to be dyed has either no affinity for the colouring matter, or not sufficient power to retain it, the combination is effected, or strengthened, by the intervention of a third substance, called a _mordant_, or basis. The mordant must have a strong affinity both for the colouring matter and the substance to be dyed, by which means it causes them to combine and adhere together. CAROLINE. And what are the substances that perform the office of thus reconciling the two adverse parties? MRS. B. The most common mordant is sulphat of alumine, or alum. Oxyds of tin and iron, in the state of compound salts, are likewise used for that purpose. _Tannin_ is another vegetable ingredient of great importance in the arts. It is obtained chiefly from the bark of trees; but it is found also in nut-galls, and in some other vegetables. EMILY. Is that the substance commonly called _tan_, which is used in hot-houses? MRS. B. Tan is the prepared bark in which the peculiar substance, tannin, is contained. But the use of tan in hot-houses is of much less importance than in the operation of _tanning_, by which skin is converted into leather. EMILY. Pray, how is this operation performed? MRS. B.