It will hardly be necessary, after all that has been already said elsewhere, to point out that the prime assistant of the chemist, water, acts in the living laboratory exactly as it does in the artificial ones. It loosens, dissolves, and mingles together the various substances which are to act upon one another. It is in the dissolved food, and the liquid blood, which flow into all the chambers and vessels of the living body, and which build up in them the fibres of living structure, and then transform and destroy those fibres, in order that the power there stored away may be obtained.
But it is still more remarkable that the other prime assistant of the chemist, fire, should also be employed in the living frame, in loosening its particles, and in quickening the operation of the various changes of substance upon which the production of power depends. It has been shown that in the body of a full-grown man there is as much heat produced in a single day, as would serve to make eighty pints of cold water boil, and it has also been stated that this heat is produced in the body exactly in the same way heat is produced in the steam engine; that is, by the burning of fuel. The heat is set free by the change of condition in material substance, precisely as power is procured. When the water employed in a steam engine is made to boil, the heat that causes the boiling issues from the coal, because that substance ceases to be coal, and turns into smoke and vapor. Just so the fuel substance of the body ceases to be fuel substance, starch, sugar, and fat—and turns into vapor, which is steamed away, leaving the heat which was concealed in the substance to warm the frame.
The furnace which is kept burning in the living laboratory, to quicken all the operations which are being carried forward in it, and to furnish its strength, is a slow and gentle one. It never burns quickly enough to cause light and flame, as common fires do. The body is never even raised to the heat of boiling water, which is far less than that of burning coal. It is only made of blood-heat; that is, sixty-eight degrees of the heat-scale warmer than freezing water,—in its warmest parts. Boiling water is one hundred and eighty degrees of the same heat-scale warmer than freezing water. The furnace of the living body sometimes burns a little more quickly than it ought, then the body gets warmed into fever. Occasionally it burns considerably less quickly than it ought, then the body is chilled, and its living actions and powers are slothful and languid. Upon the whole, however, its heat is steadily kept up at pretty much what it ought to be, that is, at one hundred degrees of the scale, which gives thirty-two degrees for frost.