The butter of cows is usually yellow: if white, its quality is inferior. If the milk has been kept too cool, the butter is pale, with little flavour, and not unctuous or rich. To have butter in perfection, the first drawn milk should be separated, and the first risen cream preferred. Some little acidity must take place previous to the separation of butter, and this must be produced by the agitation, if not before approaching. The cream should, therefore, be kept for some time previous to the churning. The yellowness of butter is probably owing to the constitution of the animal, but the contact of the air has also some effect, for the internal parts of the mass" are whiter than the external. To preserve butter from rancidity, two drachms of sugar, as much nitre, with half an ounce of salt, will be sufficient for sixteen ounces of butter. The rancidity of butter depends, in part, upon its retaining some whey, which holds a caseous matter; for it keeps in proportion to its washing: but, after every care, some of the caseous matter seems to remain, and to this its consistence is owing (Fourcroy). Thus, to melt butter without granulations, we add flour to prevent the cheesy portion falling to the bottom, and keep it in constant agitation.

Skimmed milk still retains a portion of the oil and the cheesy matter, which separate on its becoming acid; it is rich and pleasant. This matter is separated also in pellicles, by boiling. When flakes of cheese are put into a fluid alkali they are dissolved, while a considerable quantity of ammonia is separated, formed by a decomposition of the cheese, and the subsequent union of its hydrogen and azote. The alkaline solution of cheese, when heated, becomes brown, and deposits a portion of animal matter. When this matter, held in solution, is separated by acids, it is black, melts in the lire like thick oil, and, when cold, is greasy; the remaining hydrogen, with the oxygen, forming oil, and, with the alkali, becoming saponaceous. When the cheese is separated from the alkali by an acid, an hepatic odour is perceived. The caseous matter is completely dissolved by vinegar, and has a greater affinity to the vegetable acids than to any other.

Whey, when fresh, is sweetish and somewhat saline; but when filtrated, pellucid. It contains some caseous matter dissolved by an acid, which is separated on the addition of an alkali, and a small portion of sugar of milk.

The milk of women is thin, of a bluish colour, of a mild sweet taste, and a pleasant odour. Its specific gravity is 1029. On exposure to air, it is covered with a very thick white matter; but, from this, no butter could be procured. On standing, the portion which separated was still more butyraceous, though still incapable of forming butter. When distilled, water, a strong empyreumatic oil, ammonia, an acid, and inflammable gas, came over. The fluid, then, which was deposited from this unctuous matter, was a butter milk; but its transparency was not changed by alcohol or acids. After evaporation, it afforded sugar of milk, with some cheese. Female milk, when skimmed, did not, in a warm temperature, coagulate in five days; but became turbid and acid. Crystals of sugar of milk were formed, and the thick mother ley afforded muriat of soda. Pellicles rose on heating, as on cow's milk. Human milk is coagulated as cow's milk, except by acids. These, even with the assistance of heat, had no effect. It was also not coagulated by alkalis, but the kali gave it a brown, a red, and at last a black, colour; lime water, a deep yellow. The earths, the neutral and metallic salts, alcohol, or the electric fluid, produced no change except on the colour; but the infusion of oak bark and sour milk, previously coagulated, produced a coagulum. The cheese which it affords is finer and more tender than that of any other milk, but does not form a mass. Sixteen hundred parts of this milk afforded 137 of cream; forty-eight of a matter resembling butter; forty-three of cheese, and 117 of sugar. Three hundred parts of this sugar afforded eighty-five of lactic acid. Human milk scarcely becomes sour after a long period. It never passes either to the vinous or putrid fermentations.

Human milk differs so essentially in different women, and even in the same woman, at different times, that the results of experiments greatly vary. Parmentier found the results so contradictory, that he suspected some deceit, and employed only milk which he drew himself. The results of his trials were, however, still contradictory. This may, perhaps, account for Dr. Clark's assertion, that human milk contains no caseous part, and he could not succeed in coagulating it, by any means, or in any temperature. (Transactions of the Royal Irish Academy for 1788.) What has been supposed, therefore, to be a coagulum of the caseous part, may have been only cream; and during the period of the first and second colustrum only was it found yellow. At every other time it is white.

The milk of the ass is whitish, with some degree of transparency, of a peculiar smell, and a saltish taste, mixed with its sweetness. Its specific gravity is 1023. Alcohol, the metallic salts, rennets, and all the acids, except the fluor, and the cream of tartar when cold, coagulate it. Alkalis produce a slight coagulation, and different changes of colour Neutrals render it thinner. It coagulates with difficulty when at rest, and the coagulum is weak. The cream is of a yellowish white, sweet, and at first thin. Afterwards it acquires a greater consistence. The whey is sweetish and yellowish. Sixteen hundred parts of this milk afford forty-seven of cream; fifty-three of cheese; seventy-two of sugar of milk, which contains about one fourth of acid. The coagulum of 'this milk does not depend on its caseous matter, for this is spontaneously separated, falling to the bottom in the form of very tenacious particles. The cream is neither thick nor copious, and, with difficulty, assumes the form of butter, which is soft and white, without any peculiar taste, but quickly becoming rancid. The butter milk, which has a mild pleasing taste, must be carefully separated, or it soon again dissolves the butter. The sugar is in small proportion, and it contains also a little calcareous muriat, sometimes muriat of soda. It agrees with human milk in being soon converted into whey when the caseous matter is deposited; and in proportion to this deposition the sweetness increases.