This carbonic acid is formed by the union of oxygen with _carbon_ or _charcoal_, which forms a large portion of the food. Watery vapor is also formed in the capillaries by the union of oxygen with the hydrogen contained in the food and drink. During this process in the capillaries, the bright red blood of the arteries changes to the purple blood of the veins, which is carried back to the heart, to be sent to be purified in the lungs as before described. A portion of the oxygen received in the lungs unites with the dissolved food sent from the stomach into the blood, and no food can nourish the body till it has received a proper supply of oxygen in the lungs. At every breath a half-pint of blood receives its needed oxygen in the lungs, and at the same time gives out an equal amount of carbonic acid and water. Now this carbonic acid, if received into the lungs undiluted by sufficient air, is a fatal poison, causing certain death. When it is mixed with only a small portion of air, it is a slow poison, which imperceptibly undermines the constitution. We now can understand how it is that all who live in houses where the breathing of inmates has deprived the air of oxygen, and loaded it with carbonic acid, may truly be said to be poisoned and starved; poisoned with carbonic acid, and starved for want of oxygen. Whenever oxygen unites with carbon to form carbonic acid, or with hydrogen to form water, heat is generated. Thus it is that a kind of combustion is constantly going on in the capillaries all over the body. It is this burning of the decaying portions of the body that causes animal heat. It is a process similar to that which takes place when lamps and candles are burning. The oil and tallow, which are chiefly carbon and hydrogen, unite with the oxygen of the air and form carbonic acid and watery vapor, producing heat during the process. So in the capillaries all over the body, the carbon and hydrogen supplied to the blood by the food unite with the oxygen gained in the lungs, and cause the heat which is diffused all over the body. The skin also performs an office similar to that of the lungs. In the skin of every adult there are no less than seven million minute perspirating tubes, each one-fourth of an inch long. If all these were united in one length, they would extend twenty-eight miles. These minute tubes are lined with capillary blood-vessels, which are constantly sending out not only carbonic acid, but other gases and particles of decayed matter. The skin and lungs together, in one day and night, throw out three-quarters of a pound of charcoal as carbonic acid, besides other gases and water.