_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 "ice-cream" 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 surrounding 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 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. -=-=-=-=-=-=-=-=-=-=-=-= 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? -=-=-=-=-=-=-=-=-=-=-=-= 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.