The first class of proximates (starch, sugar, gum, etc.), perform an important office in the animal economy aside from their use in making fat. They constitute the _fuel_ which supplies the animal's fire, and gives him his _heat_. The lungs of men and other animals may be called delicate _stoves_, which supply the whole body with heat. But let us explain this matter more fully. If wood, starch, gum, or sugar, be burned in a stove, they produce heat. These substances consist, as will be recollected, of carbon, hydrogen, and oxygen, and when they are destroyed in any way (provided they be exposed to the atmosphere), the hydrogen and oxygen unite and form water, and the carbon unites with the oxygen of the air and forms carbonic acid, as was explained in a preceding chapter. This process is always accompanied by the liberation of _heat_, and the _intensity_ of this heat depends on the _time_ occupied in its _production_. In the case of decay, the chemical changes take place so slowly that the heat, being conducted away as soon as formed, is not perceptible to our senses. In combustion (or burning) the same changes take place with much greater rapidity, and the same _amount_ of heat being concentrated, or brought out in a far shorter time, it becomes intense, and therefore apparent. In the lungs of animals the same law holds true. The blood contains matters belonging to this carbonaceous class, and they undergo in the lungs the changes which have been described under the head of combustion and decay. Their hydrogen and oxygen unite, and form the moisture of the breath, while their carbon is combined with the oxygen of the air drawn into the lungs, and is thrown out as carbonic acid. The same consequence--heat--results in this, as in the other cases, and this heat is produced with sufficient rapidity for the animal necessities. When an animal exercises violently, his blood circulates with increased rapidity, thus carrying carbon more rapidly to the lungs. The breath also becomes quicker, thus supplying increased quantities of oxygen. In this way the decomposition becomes more rapid, and the animal is heated in proportion. Thus we see that food has another function besides that of forming animal matter, namely to supply heat. When the food does not contain a sufficient quantity of starch, sugar, etc., to answer the demands of the system the _animal's own fat_ is carried to the lungs, and there used in the production of heat. This important fact will be referred to again. FOOTNOTES: [G] By _proximate principle_, we mean that combination of vegetable elements which is known as a vegetable product, such as _wood_, etc. [H] _Muscle_ is _lean meat_, it gives to animals their strength and ability to perform labor. [I] This, of course, supposes that the soil is fertile in other respects. CHAPTER VII. LOCATION OF THE PROXIMATES AND VARIATIONS IN THE ASHES OF PLANTS. [Of what proximate are plants chiefly composed?