(From Ickak, to lick up, Arabic). Milk is the secreted fluid destined for the nourishment of the animals arranged by Linnaeus in the class of mammalia, comprehending also, from this circumstance, the cetacea. It is a white, opaque fluid; and, when viewed with a microscope, globules, like those of the blood, appear to swim in it. It is, therefore, most certainly, not an homogeneous fluid, whose parts are chemically united] but an heterogeneous one simply mixed. Its resemblance to the blood is striking; and as it is of the colour of the chyle from which the blood is formed, it was an obvious suggestion that the milk was only chyle again separated from the general mass. Chyle has not, however, been sufficiently analyzed to support or confute this idea. It appears, on the whole, impro-bable, since sugar contains a saccharine matter, and particularly a larger proportion of phosphorated lime than any of the other animal fluids. To which may be added that milk requires the digestive process as well as other nourishment; and it is never apparently assimilated until it has been coagulated.

Milk was probably the food of the earliest inhabitants of the globe, since their herds, of which their riches consisted, must have afforded an obvious supply of this delicious nourishment. Cheese seems to have been known more early than butter, for it was of importance to preserve a.supply of food when the milk of the herds would, at least lessen, if not disappear. Butter is obscurely hinted at by Herodotus, but described more particularly by Hippocrates: each author speaks of the art of making it as being derived from the Scythians. When first drawn, milk has a faint smell, generally mixed with that of the animal which afforded it. This aroma is soon lost on exposure to the air, and much of the delicacy, perhaps the salubrity, of the fluid, seems to disappear with it. The sweetishness, however, remains, but is in different degrees in different animals, depending on the proportion of sugar which it contains. The sweetest milk is that of the sheep.

When milk is at rest, first the aroma disappears, and soon afterwards the surface assumes a yellower colour, and a thick tenaceous scum called cream rises to the top. When this is separated, the milk is of a bluish white colour; and, on standing longer, the curd or cheesy-part separates. An oily substance forms a portion of the milk when first drawn; but the consistence of cream is owing to its imbibing a portion of oxygen from the air; and the curd does not spontaneously separate till the acid fermentation begins. It is separated, artificially, by a variety of substances, as all the acids, except the carbonic, and the weaker kinds similar to it; by different vegetables, as the galium, the vallantia cruciata, the madder, the bark, and, apparently, other vegetable astringents; by some animal substances, as the stomach of a young animal, however carefully washed and dried, the livers of turkeys (Spallanzani), etc. Jacquin, in his Elements of Chemistry, tells us, that the vegetables only act when cold, or in cold infusions. When boiled in milk, or boiling decoctions of the same plants are added, coagulation is retarded rather than hastened. Milk is coagulated also by salts, particularly such as contain an excess of acid, as the cream of tartar; benzoic and succinic salts; by metallic solutions; by alcohol, and all spirituous liquors; though the addition of camphor or borated soda will, it is said, prevent this effect when either is dissolved in alcohol. In every instance, however, the coagulation is firmer and more perfect with the assistance of heat.

The alkalis which are said to coagulate milk unite in reality with the oily part, and produce a soap, which seems to entangle, occasionally, some of the curd. These flocculi, for such is their form, become successively, by boiling, yellow and brown. Pure alkalis render the milk more fluid, by equally dissolving the oil and the curd. Lime water seems to procure an imperfect coagulation.

Milk, when urged by heat, gives up its oily portion, which forms a dense pellicle; and that part of it which touches the sides of the heated vessel burns, and gives the whole an empyreumatic taste. If this is prevented, the serum procured is thin and pure. If, however, the heat is more violent, an insipid water comes over, flavoured with the aroma of the milk, which soon becomes putrid. The remainder is an extract, which, with warm water, again becomes milk, though without the aroma. If this extract be exposed to a strong heat, an empyreumatic acid oil, ammonia, hydrogenous and carbonic acid gases come over. The remaining coal affords kali, muriated kali, phosphat of lime, and occasionally a little iron.

When left untouched, milk undergoes the acetous fermentation at different periods, according to the heat of the weather and the nature of the animal. In warm weather, and in ruminating animals, this change is soonest observed. Mare's milk continues longest unaltered. The tendency of milk to the acetous fermentation is checked, it is said, by boiling.

If milk in a moderately warm place be frequently stirred, no separation occurs, and the vinous fermentation comes on. The greatest quantity of spirit, it is said, will be afforded by cow's milk, though that of the mare, as the most saccharine fluid, ferments soonest; and the Tartars prefer it, as affording also more spirit. The putrefactive process comes on slowly. Stipriaan (Memoires de la Societe de Medecinea Paris, 1787-8) informs us, that cow's milk showed no signs of putridity after four summer months, asses milk three months, and female milk nearly an equal time.

The cream, we have said, is the oil, which has acquired a greater consistency by its union with oxygen. In the form of butter it is still more intimately united with this principle, and a chemical union apparently takes place as heat is excited. In general, the cream is suffered to rise spontaneously, but in the West its separation is assisted by heat. The milk is put in shallow earthen pans, and remains in them twelve hours in summer, and twenty-four in winter. The pans are then placed on hot stoves, and the temperature raised, so as to be scarcely short of boiling. On the first appearance of bubbles the pans are removed, and remain at rest twelve or twenty-four hours longer, according to the season. This cream, styled scalded or clotted, is generally agitated by the hand in making butter, and the churn is only used when the raw cream is employed.