As no part of the oxygen taken into the system is given off in other combination than with carbon and hydrogen, and as the carbon and hydrogen must be replaced by the food taken into the system, it is clear that the nourishment required must be in a ratio with the amount of oxygen taken into the body. Two animals which take up an unequal amount of oxygen require in the same ratio an unequal amount of food. The child whose respiration is very active, requires food much oftener than an adult. Deprive a bird of food and it dies in a few days, while a serpent, with its slow respiration, will live without food two or three months. The quantity of oxygen we respire is materially affected by the atmosphere, its density and change of temperature. In summer the air is not only rarefied, but contains a vast amount of aqueous vapor, while in winter it is not only dry, but condensed by cold. Thus the same volume of air contains more oxygen in cold northern climates than in tropical regions, in winter than in summer. The inhabitants of a warm climate therefore would require far less carbon to support life than those who are compelled to endure the intense cold of the frigid and upper portion of the temperate zones. For the same amount of force far more oxygen is inspired in cold than in warm weather, on the sea-shore than on the summit of high mountains. Nature, with the wisdom which governs all her movements, has so ordained, that the fruits and vegetables which compose a large proportion of the food in tropical climates should contain far less carbon than the food in colder regions, notwithstanding a larger amount in bulk may be used by the former than the latter. The fruit on which the natives of warm climates mostly feed, contains but about 12 per cent. of carbon, while the meat and train-oil in arctic regions contain from 66 to 80 per cent.

From what has been stated, it will readily be perceived, that the true source of animal heat is the action between the elements of food and the oxygen conveyed to every part of the body by means of the blood. Carbon cannot combine with oxygen without heat being evolved. The amount of heat of course varies with the amount of oxygen introduced into the system. The temperature of a child whose respiration is very rapid, is higher than that of an adult; the former being 102°, the latter 99.5°. The heat of an adult is the same in every part of the world, where the thermometer continues for weeks at 90° above zero and where it ranges for months from twenty to forty below, amid the ice which encircles the poles, and the rich vegetation which grows with amazing rapidity beneath a tropical sun. This will not appear strange when we look upon the animal body as a heated mass, giving out heat where the surrounding objects are colder than itself, and taking it in where they are warmer. What an enormous difference there must be in the amount of heat given out in those warm climates where the temperature is nearly the same as the body and in those intensely cold regions where it is 100° lower, and what a vast difference there must also be in the amount of food required to keep up that combustion which produces animal heat. Deprive the native of the south of food, and death would be slow in its progress. Deprive the inhabitant of a frozen region of food, and death would be speedy. The enormous combustion required to be kept up to defend the system against the cold would in a short time exhaust the body of its carbon and death would ensue. There are many tribes of savages who go nearly naked where the climate is intensely cold, but they consume a vast amount of carbon, in the form of meat, train-oil and tallow. Clothes after all are only an equivalent for a certain amount of food. Food is regulated by the number and strength of the respirations, the temperature of the air and the amount of heat given off. The amount of oxygen consumed, depends upon the temperature and density of the air, motion, and the amount and quality of food consumed.

Without detriment to health no more carbon and hydrogen can be taken into the system than is given off in carbonic acid and water, and yet persons accustomed to large quantities of substantial food, on visiting a warm climate, think they must eat the same articles and as much of them as they did when the thermometer was thirty or forty degrees less. On finding their appetite flag and its relish for hearty food subside, they stimulate it, by means of cayenne pepper, mustard, brandy, wine, and a hundred other stimulants. The consequence is, an unnatural state is induced, the carbon is not consumed, disease ensues, and the patient stands a fair chance of paying for his ignorance and folly, with his life. The evils resulting from an abuse of intoxicating drinks have been sketched in such graphic colors as to almost curdle the blood with horror, and yet the question would be by no means a difficult one to decide - which has created more pain, and caused more deaths, intoxicating drinks or errors in diet?

If we hold that the increase of mass in the animal body, the development of organs, and the supply of waste, is dependent on the blood, to ascertain whether a substance contains nourishment, all that is necessary for us to do, is to compare its ingredients with those of the blood.

The chief ingredients of blood will be found to be fibrin and albumen. Blood, after having been drawn from the body, coagulates and separates into a yellowish liquid, called the serum, and a gelatinous mass, adhering to a rod on stirring it, in soft elastic fibres. This is called fibrin, and is identical with a muscular fibre purified from foreign matter. The albumen is contained in the serum and gives to it the properties of the white of eggs; when heated it coagulates into a white elastic mass.

Fibrin and albumen are shown by chemical analysis to contain the same ingredients in the same proportion. The particles are arranged however in different order, as is shown by the difference of their external properties. They also contain the earth of bones. The serum retains in solution sea salt, and other salts of potash and soda in which the acids are carbonic, phosphoric and sulphuric. The globules of the blood contain not only fibrin and albumen but a red coloring matter, in which iron is an element. Both albumen and fibrin in the process of nutrition are capable of being converted into muscular fibre, and muscular fibre is capable of being converted into blood.