The aliments or nutritious principles of food are divided into two great classes: the one is distinguished by the absence of the element nitrogen, and is termed non-nitrogenized ; the other containing that element, and called ni-trogenized aliments. The first or non-nitro genized, contain the elements carbon, hydro gen, and oxygen, and are divided into three groups, depending upon the relative propor tion of these elements. Sugar and starch are distinguished by possessing an identical pro portion of carbon, and by having an equal number of hydrogen and oxygen atoms, these elements being in the exactproportion to form water. Starch consists of 12 atoms of carbon or charcoal, and 10 of water. Vegetable acids—those substances which impart sour ness to fruits—contain variable proportions of carbon, a very small amount of hydrogen, and an excess of oxygen. Fats and oils are found to be composed almost entirely of carbon and hydrogen, with but a very small amount of oxygen. Albumen, iibrine, caseine, whether derived from the animal or vegetable world, are identical in composition. Thus we have vegetable and animal albumen, vegetable and animal fibrine, and vegetable and animal ca seine, accordingly as they are obtained from either animals or plants. None of these sub stances will sustain life alone; this has been often proved. They must be mingled toge ther. Man requires a mixed diet, but how shall it be mixed. Here, fortunately, nature comes to our aid—she has prepared a recipe. We find it in the composition of milk, the true type of all diet. Here the alimentary ingre dients are so judiciously mingled, as to lur-nish the elements for forming the entire hu man body. The true philosophy of dietetics is to be found in a milk pail. Heat, the agent used in cooking, is very poweriul to alter and destroy all organized compounds. It is va riously applied, as iu boiling, roasting, frying, and with different results, both in kind and degree. Many of these changes are not yet well understood—they have not been suffi ciently studied. Of all the alimentary princi ples, albumen is the most promptly altered by heat. It exists in vegetables and also in ani mals—in their blood, and also diffused through out the flesh in a liquid form, in which it dis solves in water. A small amount of heat converts it into a hard, brittle solid, insoluble in water, forming coagulated albumen, as in the boiled white of eggs. If, therefore, a piece ot fresh meat is placed in cold water, the albu men tends to dissolve out—it is withdrawn from the flesh. If the meat is put in boiling water, the albumen, on the contrary, coagu lates all over the surtace, forming a crust which cuts off the solvent action of the wa ter. To retain the albuminous juices in substan ces to be boiled, they should be added to the boiling water ; if, on the contrary, we wish to extract these juices, as in making poup, an op posite proceeding is admissible—we add the solids to cold water and gradually raise the temperature. Prof. Leibig says, that in salt ing meat, the brine which is formed contains a large proportion of the most nutritious jui-c es of the flesh. By salting, therefore, the normal composition of meat is essentially al tered. The necessity of food arises from the waste and wear of all parts of the system. As the body is used its atoms die and are car ried away. As the dead atoms perish, new ones must be constantly supplied of different kinds, as the various parts of the body may require. The body, therefore, analyses the food that is taken into it. It separates it into its elements, withdrawing one part here and another there, as it needs them for different purposes and in different places. But before these constituents of food can be separated, it must first be dissolved, just as a chemist must first dissolve a mineral before he can separate its elements or analyze it. This solution of food is called digestion. It begins mechani cally in the mouth by mastication, just as the chemist first crushes his mineral with pestle and mortar. It is then carried into the sto mach, which pours from its wells a liquid known as gastric juice, with which it is mingled by a pecnliar agitating motion of that organ. The active principles of th gastric juice are acids, chiefly muriatic with perhaps lactic and phosphoric, and a peculiar organic principle termed pepsin. It is always distinct ly acid : this liquid attacks and dissolves the nitrogenized alimentary principles, the other class remains unchanged and untouched in the stomach. Stomach digestion by no means completes the process. It dissolves only al buminous substances, a portion of which is immediately absorbed by the veins of the eto-mach and carried at once into the circulating system. The residue of the food now passes forward into the first portion of the intestine called the duodenum. Into the duodenum there is poured from the liver a liquid called bile, and from the pancreas another liquid called pancreatic juice, both alkaline from the presence ot soda in considerable quantities. These alkaline juices now seize upon the un-dissolved alimentary compounds, starch, su gar and the oily bodies,dissolve and transform them, thus completing the act of digestion. It is but recently that Bernard of France has demonstrated that the office of the pancreatic juice is to dissolve the fatty substances, and for this demonstration he lately received the prize of the Paris Academy of Sciences. The dissolved portions of the food which are to become blood are now taken up from the in testine by innumerable little vessels termed lacteals, which carry forward their contents and deliver them into a large vein in which they are swept along into the great current of the circulation. It may be regarded as a physiological fact, settled beyond reasonable doubt, that the destination of the non-nitroge nized alimentary principles is to be burned throughout the body by oxygen introduced in respiration ior the maintenance of animal tem perature. These substances evidently cannot be converted into the tissue of the fabric, for they do not contain the materials to form that tissue. They are of various degrees of com-bustility, giving rise to unequal amounts of heat by burning, and are therefore adapted to different climatic and seasonal conditions of temperature. In the colder regions, foods rich in hydrogen and carbon are instinctively sought. In the warmer climates, the less combustible starches and vegetable acids are prized. On the other hand, nitrogenized ele ments minister to the true nourishing process —they are transformed into muscle and tis sue ; they .build up the labric. All these or ganized substances are designed to be decom posed in the production and evolution of pow er. Now, if there be a limit to the power of vegetable construction upon a given area of land, there is also a limit to the number of animals that can live upon that area. The agricultural or grain-consuming races are by far the most powerful and are rapidly driving the hunting or flesh-eating races from the face of the earth.