Milk is an opaque fluid, generally white in color, having a sweet and agreeable taste, and is composed of a fatty substance, which forms butter, a caseous substance, which forms cheese, and a watery residuum, known as serum, or whey, in cheese-making. The fatty or butyraceous matter in pure milk varies usually from two and a half to six and a half per cent.; the caseous or cheesy matter, from three to ten per cent.; and the serous matter, or whey, from eighty to ninety per cent. To the naked eye milk appears to be of the same character and consistence throughout; but under the microscope a myriad of little globules of varied forms, but mostly round or ovoid, and of very unequal sizes, appear to float in the watery matter. On more minute examination, these butter-globules are seen to be enclosed in a thin film of caseous matter. They are so minute that they filter through the finest paper. Milk readily assimilates with water and other sweet and unfermented liquids, though it weighs four per cent. more than water. Cold condenses, heat liquefies it. The elements of which it is composed, not being similar in character or specific gravity, undergo rapid changes when at rest. The oily particles, being lighter than the rest, soon begin to separate from them, and rise to the surface in the form of a yellowish semi-liquid cream, while the greater specific gravity of the serous matter, or whey, carries it to the bottom. A high temperature very soon develops acidity, and hastens the separation of the cheesy matter, or curd, from the whey. And so the three principal elements are easily distinguished. But the oily or butyraceous matter, in rising to the surface, brings up along with it many cheesy particles, which mechanically adhere to it, and give it more or less of a white instead of a yellow color; and many watery or serous particles, which make it thinner, or more liquid, than it otherwise would be. If it rose up free from the adhesion of the other elements, it would appear in the form of pure butter, and would not need to undergo the process of churning to separate it from other substances. The time may come when some means will be devised, either mechanical or chemical, to separate the butter particles from the rest instantaneously and completely, and thus avoid the often long and tedious process of churning. The coagulation, or collecting together of the cheesy particles, by which the curd becomes separated from the whey, sometimes takes place so rapidly, from the effect of great heat, or sudden changes in the atmosphere, that there is not time for the butter particles to rise to the surface, and they remain mixed up with the curd. Nor does the serous or watery matter remain distinct or free from the mixture of particles of the cheesy and buttery matters. It also holds in suspension some alkaline salts and sugar of milk, to the extent of from three to four per cent. of its weight. We have, then,