A knowledge of the constituents of milk may aid the farmer in selecting the substances proper for the nourishment of animals, and promotive of the lacteal secretion; for much of the food contains those materials united, though not always in the same form. "The constituents of milk are cheese, or caseine--a compound containing nitrogen in large proportion; butter, in which hydrogen abounds; and sugar of milk, a substance with a large quantity of hydrogen and oxygen in the same proportions as in water. It also contains, in solution, lactate of soda, phosphate of lime, (the latter in very small quantities,) and common salt; and a peculiar aromatic product exists in the butter, called butyric acid."--_Liebig._ It is very difficult to explain the changes which the food undergoes in the animal laboratory, (the stomach,) because that organ is under the dominion of the vital force--an immaterial agency which the chemist cannot control. Yet we are justified in furnishing the animal with the elements of its own organization; for although they may not be deposited in the different structures in their original atoms, they may be changed into other compounds, somewhat similar. Liebig tells us that whether the elements of non-azotized food take an immediate share in the act of transformation of tissues, or whether their share in that process be an indirect one, is a question probably capable of being resolved by careful and cautious experiment and observation. It is possible that these constituents of food, after undergoing some change, are carried from the intestinal canal directly to the liver, and that there they are converted into bile, where they meet with the products of the metamorphosed tissues, and subsequently complete their course through the circulation. This opinion appears more probable, when we reflect that as yet no trace of starch or sugar has been detected in arterial blood, not even in animals that have been fed exclusively with these substances. The following tables, from Liebig's Chemistry, will give the reader the difference between what is taken into the system and what passes out. FOOD CONSUMED BY A COW IN TWENTY-FOUR HOURS.