The thickest and richest creams are afforded by the sheep and goat; the milk of the mare, the ass, and the female, afford the thinnest. From female milk scarcely any separation takes place, even with the assistance of heat. The fluid separated in making butter is called buttermilk. It is the serum, enriched with some of the oil of the cream.

The curd is a true albuminous substance, without smell or taste, nearly insoluble in water, hardened in hot water, soluble in acids, forming, with the vitriolic and marine, brown solutions; with the nitrous a yellow. It is easily dissolved by alkalis, but most powerfully by the mineral; and, when this is pure, during the solution, a volatile alkaline smell arises; a fact not sufficiently noticed, and which will admit of some application. In distillation, an insipid water comes over that easily putrefies; and the remainder, on increasing the heat, blisters, like burnt horn, affording hydrogenous and carbonic acid gas, ammonia, a heavy fetid empyreu-matic oil. The coal contains lime and phosphat of lime. When exposed to a strong heat, in an open fire, curd softens and melts; becomes transparent and tough; and, when cold, is hard and brittle.

The curd of goat and cow's milk is solid and elastic; that of the ass's and mare's milk less solid; of the sheep merely glutinous; of the female generally fluid, and with difficulty separated. See Caseus.

The whey is similar to the serum of the blood. The whey separated in making cheese is a watery fluid, without any admixture of oil; but, in its usual state, some of the oily, and some of the albuminous portion are diffused through it. Sweet whey affords sugar of milk: when the milk has been previously sour, it is styled sour whey; and, by adding a small portion of spirit of wine, and subjecting it to a fresh fermentation, a true vinegar of milk may be prepared (Jacquin). The spirit of milk is apparently not developed till it becomes acid; the Tartars hasten the acetous fermentation by the addition of oatmeal, and do not distil it till it is strongly sour. Thus milk holds a middle place between animal and vegetable substances. As it undergoes the acetous and vinous fermentations, and becomes very slowly putrid, it resembles vegetables. Its albuminous curd is of an animal nature.

Before we proceed to consider the medical properties of milk, we shall describe more particularly the milk of different animals, viz. that of the cow, the human female, the ass, the goat, the sheep, and the mare.

The general appearance of corn's milk is well known, and to this standard we shall refer the taste and more obvious properties of other milks. It is differently flavoured, however, according to the age and the food of the animal. When near the time of calving, it is more of an animal nature than at a future period; and the milk of farrow cows is, at first, saline. The alliaceous and the umbelliferous plants, horse mint, cabbages, and turnips, give it their peculiar flavour. The leaves of maize are said to render it mild and saccharine; the po-tatoe plant insipid. The Alderney, Alpine, and Sardinian cows give a very rich milk: those of the north and of Catalonia an aqueous blue milk. Tessier observed some milk, which was white when drawn, soon became blue; a change attributed to their eating the isatis (woad). Dr. Garden found, that the milk was blue after the cows had eaten of this plant; and it is usually red when they have eaten madder or the fruit of the cactus opuntia. Cow's milk, in an ordinary state, boils at 113°.

The carbonic acid gas, the boracic and oxalic acids, do not coagulate this milk when mixed with it, though Stipriaan observes that the latter, if strewed on it in powder, will have this effect. The curds produced by the other acids, are dissolved again by alkalis. When vinegar is employed for the coagulation, the dissolved curd is of a rose colour. Other re-agents acton it nearly as we have said they do on milk in general. The electric fluid thickens it a little. Its specific gravity is 1028.

Eight pounds of milk afforded, on distillation, eight ounces of a clear fluid, which retained the smell and taste of the animal's food. This fluid became turbid after some time, but again clear in a heat of twenty-eight of Reaumur, while some light filaments were formed. When filtered it became clear and tasteless, leaving nothing after distillation. The residuum after the first distillation was sweet and butyraceous, called by Hoffman franchipan; and, when diluted in boiling water, the white milky fluid is called Hoffman's whey. When distilled, it affords water, a very fluid yellow oil, an acid, a volatile alkali, a thick black empyreumatic oil, and an inflammable gas. The ashes seem to contain an alkali and a muriated soda.

The first milk after calving (colostrum, primum, or beastings, ) resembles other milk, with the addition of a mucous substance. The specific gravity is 1072. It is yellow, but soon coagulates on the fire, and becomes white. The specific gravity of the second colostrum was 1052. It is more difficult to coagulate, and stirring wholly prevents this change. Rennet changes the first colostrum to a jelly, and coagulates the second: the first contains more than three times the quantity of butter, and a much larger proportion of albuminous matter. Both colostra become more quickly putrid than other milk, and seem to contain a large proportion of the gluten of the blood.

The cream of cow's milk after standing about three weeks, was covered with a green effervescence, and the under part had a cheesy flavour (Parmentier). The scalded cream acquires this flavour much sooner; and this was the reason for our asserting that some of the curd was probably entangled with it. On distilling this semiputrid cream, the usual results obtained from fat bodies were discovered. (See Adeps.) The milk, last drawn from the cow, has the largest proportion of cream; and the cream, as it rises in succession on the milk, while at rest, decreases in quantity, and deteriorates in quality. Thick milk throws up less cream than that which is thinner, but its quality is better. If the milk is diluted with water, the cream is more copious, but less rich. Previous agitation lessens the proportion of cream; and the proper temperature for its rising most successfully is about 54° of Fahrenheit. Milk in autumn gives more cream than in spring; but it rises soonest in summer.