if its capacity for caloric is lessened, heat will be set free and given out to surrounding bodies.

Illustrations. 1st. Ice and salt, (Chl. of Sodium,) when mixed, are converted into a fluid. In this state they will hold more heat than when solid. The heat necessary to produce this change is drawn from the surrounding medium, which is made proportionally colder by the loss of caloric imparted to the ice and salt. It is by this chemical process that "icecream is made.

2d. On the other hand, mix water and sulphuric acid, (oil of vitriol,) of the temperature of 60°, and the mixture will become quite warm, and will freely impart its heat to surreunding and contiguous objects.

562. The same principle is exhibited, when oxygen unites with an inflammable body, as in the burning of wood. coal, oil, &c. In combustion, the oxygen of the atmosphere unites with carbon and hydrogen, and carbonic acid and water are produced. This process, according to all the known laws of caloric, is attended with heat. The quantity of heat disengaged in combustion is always in proportion to the amount of carbon and hydrogen consumed; thus a piece of wood weighing one pound, in burning slowly, would give out the same quantity of heat as a pound of shavings of the same wood, in burning rapidly. Upon these principles, the production of animal heat may be understood.

563. The food contains carbon and hydrogen. These exist in the chyle. The old and waste atoms of the body likewise contain the same elements. In the lungs the oxygen and nitrogen of the inspired air are separated. It is now supposed that the oxygen enters the capillary vessels of the

Give the 1st illustration. The 2d. 562. What changes take place when oxygen unites with an inflammable body? To what is the quantity of heat proportionate in combustion? Give an example. 563. How are carbon and hydrogen supplied to the system? How the oxygen? Where does the oxygen mingle with the blood?

lungs, and mingles with the blood, with which it is carried to the heart and thence to the nutrient capillary vessels of every part of the system.

564. In the capillary vessels, the oxygen of the arterial blood unites with the carbon and hydrogen which the refuse materials contain, and carbonic acid and water are formed. The combustion of carbon and hydrogen in the capillaries of every part of the system, (the lungs not excepted,) is attended with a disengagement. of heat, and the carbonic acid and water are returned to the lungs in the dark-colored blood, and evolved from the system.

565. Sir Benjamin Brodie and some others have inaintained, that the heat of the system is generated exclusively by the influence of the brain and nerves. This theory is discarded by most physiologists; yet it is true that the nervous system exercises a great influence over the action of the capillary vessels in the process of nutrition, secretion, and absorption. When these operations are most active, the change among the particles of matter of which the body is composed, is then greatest, and the generation of heat is increased in a corresponding degree.

566. The necessity of pure, red blood in the production of animal heat, is shown when the vessels that carry blood to a limb are ligated, or tied; the part immediately becomes colder. The necessity of nervous influence is seen in the diminished temperature of a paralytic limb.

567. Our next inquiry is, By what means is the uniformity of temperature in the body maintained? As there is a constant generation of heat in the system, there would be an undue accumulation, so much so as to cause disagreeable

564. Where does it unite with the carbon and hydrogen contained in the body, and how is heat generated? 565. What was the theory of Sir Benjamin Brodie? Is this theory in general discarded? What is true of this theory? 566. How is the necessity of pure, red blood and nervous action shown in the production of animal heat?

sensations,

if there were no means by which it could be

evolved from the body, or its production lessened.

568. It has been ascertained that the principal means by which the system is kept at a uniform temperature, is the immense evaporation from the skin and lungs. These membranes, in an ordinary state, are constantly giving out water, which is converted into vapor, and carried off by the surrounding air. The quantity of heat abstracted from the system to effect this, depends on the rapidity of the change of air, its temperature, and the amount of water it contains in a state of vapor. The quantity removed is greatest when the air is warm and dry, and the change, or current, rapid.

Observations. 1st. The first discovery of the use of free evaporation of the perspiration from the skin in reducing the heat of the body, and the analogy subsisting between this process and that of the evaporation of water from a rough porous surface, so constantly resorted to in warm countries, as an efficacious means of reducing the temperature of the air in rooms, and of wine and other drinks, much below that of the surrounding atmosphere, was made by Franklin.

2d. In all ages and climes, it has been observed that the in creased temperature of the skin and system in fevers, is abated as soon as free perspiration is restored. In damp, close weather, as during the sultry days of August, although the temperature is lower, we feel a disagreeable sensation of heat, because the saturation of the air with moisture lessens evaporation, and thus prevents the escape of heat through the lungs and skin.

3d. It is on the principle of the evaporation of fluids, that warm vinegar and water, applied to the burning, aching head, cools it, and imparts to it a comfortable feeling. The same

568. What are the principal means by which a uniform temperature of the body is maintained? On what does the quantity of heat abstracted from the system depend? What discovery relative to animal heat is due to Franklin? What is said of free perspiration in fevers? What occasions the disagreeable sensation of heat in damp, close weather?

results follow if warm liquids are applied to the skin in the hot stage of fever; and this evaporation can be increased by constant fanning.

4th. It is frequently noticed, in very warm weather, that dogs and other domestic animals are seen with their tongues out of their mouths, and covered with frothy secretions. This is merely another mode of reducing animal heat, as the skin of such animals does not perspire as much as that of

man.

569. Under some circumstances, a portion of the heat of the system is removed by radiation. When cold air comes in contact with the skin and mucous membrane of the lungs, heat is removed from the body, as from a stove, to restore an equilibrium of temperature. The removal of heat from the body is greatest when we are in a current of cold air, or when a brisk, cold wind is blowing upon us.

570. As the primary object of the different processes of nutrition is to supply animal heat, so the action of the dif ferent nutritive organs is modified by the demands of the system for heat. When heat is rapidly removed from the body, the functional activity of the organs of nutrition is increased. When the system is warmed by foreign influence, the activity of the nutritive organs is diminished. This leads to the natural, and, we may add, instinctive change in the quality and quantity of food at different seasons of the year.

569. When is heat radiated from the body? When is it greatest? 570. What is the primary object of the different processes of nutrition? When is the activity of the nutritive organs increased? When diminished ? To what does this lead?

CHAPTER XXVIII.

HYGIENE OF ANIMAL HEAT.

571. THE amount of heat generated in man and inferior animals depends upon the quantity and quality of the food, age, exercise, the amount and character of the respired air, condition of the brain, skin, and general system.

572. Animal heat is modified by the proportion of digestible carbon which the food contains, and by the quantity consumed. As the kind of fuel that contains the greatest amount of combustible material evolves the most caloric when burned, so those articles of food that contain the greatest quantity of carbon produce the most heat when converted into blood. The inhabitants of the frigid zones, and individuals in temperate climates during the cold season, consume with impunity stimulating animal food, that contains a large proportion of carbon, while the inhabitants of the tropical regions, and persons in temperate climates during the warm season, are more healthy with a less stimulating or vegetable

diet.

Observation. When we ride or labor in cold weather, an adequate amount of nutritious food will sustain the warmth of the system better than intoxicating drinks.

573. Age is another influence that modifies the generation of animal heat. The vital forces of the child being feeble, less heat is generated in its system than in that of an adult.

571-585. Give the hygiene of animal heat. 571. State some of the influences that modify the generation of animal heat. 572. What element of the food influences the generation of heat? When and where can animal food be eaten with impunity? Give the practical observation.