Animal oil is not so easily acidified as the other ingredients of milk. Butter, therefore, though usually made of sour cream, is not sour itself, because the oily part of the cream had not been acidified. Butter, however, is susceptible of becoming acid by an excess of oxygen; it is then said to be rancid, and produces the sebacic acid, the same as that which is obtained from fat. EMILY. If that be the case, might not rancid butter be sweetened by mixing with it some substance that would take the acid from it? MRS. B. This idea has been suggested by Sir H. Davy, who supposes, that if rancid butter were well washed in an alkaline solution, the alkali would separate the acid from the butter. CAROLINE. You said just now that creamed milk consisted of curd and whey. Pray how are these separated? MRS. B. They may be separated by standing for a certain length of time exposed to the atmosphere; but this decomposition may be almost instantaneously effected by the chemical agency of a variety of substances. Alkalies, rennet*, and indeed almost all animal substances, decompose milk by combining with the curds. Acids and spirituous liquors, on the other hand, produce a decomposition by combining with the whey. In order, therefore, to obtain the whey pure, rennet, or alkaline substances, must be used to attract the curds from it. But if it be wished to obtain the curds pure, the whey must be separated by acids, wine, or other spirituous liquors. [Footnote *: Rennet is the name given to a watery infusion of the coats of the stomach of a sucking calf. Its remarkable efficacy in promoting coagulation is supposed to depend on the gastric juice with which it is impregnated.] EMILY. This is a very useful piece of information; for I find white-wine whey, which I sometimes take when I have a cold, extremely heating; now, if the whey were separated by means of an alkali instead of wine, it would not produce that effect. MRS. B. Perhaps not. But I would strenuously advise you not to place too much reliance on your slight chemical knowledge in medical matters. I do not know why whey is not separated from curd by rennet, or by an alkali, for the purpose which you mention; but I strongly suspect that there must be some good reason why the preparation by means of wine is generally preferred. I can, however, safely point out to you a method of obtaining whey without either alkali, rennet, or wine; it is by substituting lemon juice, a very small quantity of which will separate it from the curds. Whey, as an article of diet, is very wholesome, being remarkable light of digestion. But its effect, taken medicinally, is chiefly, I believe, to excite perspiration, by being drunk warm on going to bed. From whey a substance may be obtained in crystals by evaporation, called _sugar of milk_. This substance is sweet to the taste, and in its composition is so analogous to common sugar, that it is susceptible of undergoing the vinous fermentation.