tissue, and consequently of the amount of muscular power generated in the animal organism.

7 The correspondence between the amount of the motive power of an animal, and the quantity of effete nitrogen excreted from the body, is limited to laboring men and to the lower animals. Strange as it may appear, it is an incontrovertible fact that men whose pursuits require the constant exercise of the intellectual faculties—lawyers, writers, statesmen, students, scientific men, and other brain-workers—excrete more urea than do men engaged in the most physically laborious occupations. An activity of thoughts and ideas involves a corresponding destruction of the tissues, and these require, for their reparation, the consumption of food. Here, then, we have a physical meaning for the common expression—"food for thought."

That the amount of heat developed in the animal organism, is proportionate to the quantity of fatty matters (or of substances capable of forming them) supplied to it in the shape of food, is a proposition which admits of easy demonstration. The natives of warm regions do not require the generation of much heat within their bodies, because the temperature of the medium in which they exist is generally as high as, or higher than, that of their blood. But as they must consume food for the purpose of repairing the waste of their nitrogenous tissues, and as every kind of food contains heat-producing elements, an excess of heat is developed within their bodies, which, if allowed to accumulate, would speedily produce fatal results. The means by which nature removes this superabundant heat are admirably simple, as indeed all its contrivances are. The skin is permeated with millions of pores, and through these openings a large quantity of vapor is given off, and carries with it the surplus heat. The pores are the orifices of minute convoluted tubes which lie beneath the skin, and when straightened measure each about the tenth of an inch, or, according to a writer in the British and Foreign Medico-Chirurgical Review (1859, page 349), the one-fifteenth of an inch in length. According to Erasmus Wilson, the number of these tubes which open into every square inch of the surface of the body is 2,800. The total number of square inches on the surface of an average sized man is 2,500, consequently the surface of his body is drained by not less than twenty-eight miles of tubing, furnished with 7,000,000 openings. The cooling of the body, by the evaporation of water from it, admits of explanation by well-known natural laws. Water, in the state of vapor, occupies a space 1,700 fold greater than it does in its liquid condition. It is heat which causes its vaporous form, but it ceases to be heat when it has accomplished this change in the condition of the liquid; for, suffering itself an alteration, it passes into another form of force—mechanical, or motive power. The heat generated within the body is absorbed by the liquid water, the conversion of the latter into vapor follows, and both the heat and the water, in their altered forms, escape through the pores.

Fatty food necessary in cold climates.—As a grave objection against the chemical theory of heat, it has been urged that rice—the pabulum of hundreds of millions of the inhabitants of tropical regions—contains an exceedingly high proportion of heat-giving substances. I have, however, great doubt as to rice ever forming the exclusive food of those people, without their health being impaired in consequence of the deficiency in that substance of the plastic elements of nutrition. Indeed I believe it is a great mistake to assert that the natives of India live almost exclusively on rice. This article, no doubt, forms a large proportion of their food, but it is supplemented with pulse (the produce of leguminous plants), which is rich in flesh-forming materials, also with dried fish, butter, and various kinds of vegetable and animal food rich in nitrogen. The innutritious nature of rice is clearly shown by its chemical composition, and so large a quantity of it must the Hindu consume in order to repair the waste of his body, that his stomach sometimes acquires prodigious dimensions; hence the term "pot-bellied," so often applied to the Indian ryot. I doubt very much, however, if the stomach of the Hindu, large as it is, could accommodate a quantity of rice, the combustion of which would produce a very excessive development of heat. This substance, when cooked, contains a high proportion of water, the evaporation of which carries off a large amount of the heat generated by the combustion of its respiratory constituents. The amount of motive power developed by the Hindu is small as compared with that which the European is capable of exerting; hence he has less necessity for a highly nitrogenous diet. On the whole, then, I am disposed to think that the food of the natives of tropical climates contains sufficient nitrogenous matters to effectually build up and keep in repair their bodies; it also appears clear to me that the amount of heat developed in their bodies is not excessive, and that it is readily disposed of in converting the water, which enters so largely into their diet, into vapor. The proportion of plastic to non-plastic elements in the diet of the Hindu and of the well-fed European, is probably as follows:—

This statement does not quite correspond with Liebig's, who estimates the proportion of nitrogenous to non-nitrogenous substances in rice as 10 to 123, in beef as ten to seventeen, and in veal as ten to one. The results of Lawes and Gilbert's investigations, already alluded to, have, however, dispelled the illusion that the plastic constituents of flesh exceed its non-plastic. In the potato, which at one time constituted more of the food of the Irish peasantry than rice does that of the Hindu, the proportion of plastic to non-plastic materials is as 10 to 110. The results of some analyses of the food grains consumed in the Presidency of Madras, made by Professor Mayer, of the University of Madras, clearly prove that the food of the inhabitants of that part of India is of a far more highly nitrogenous character than is generally supposed. That the Hindu, who subsists exclusively on rice, exhibits all the symptoms of deficient nutrition, is a fact to which numerous competent observers have testified.

A slight consideration of the facts which I have mentioned leads to the conclusion that the food of the inhabitants of very cold regions is required to produce a large amount of heat. Melons, rice, and other watery vegetable productions, however delicious to the palate of the Hindu, would be rejected with disgust by the Esquimaux, whilst the train oil, blubber, and putrid seal's flesh which the children of the icy North consider highly palatable, would excite the loathing of the East Indian. On this subject I may appositely quote the following remarks by Dr. Kane, the Arctic explorer:—"Our journeys have taught us the wisdom of the Esquimaux appetite, and there are few among us who do not relish a slice of raw blubber, or a chunk of frozen walrus beef. The liver of a walrus (awuktanuk), eaten with little slices of his fat—of a verity it is a delicious morsel. Fire would seem to spoil the curt,