By oxidation the nitrogen of animal substances is converted into nitric acid, HNO₃. Furthermore, the silent, continuous action of minute living organisms in the cell is always tending to transform the waste-products of animal life into compounds closely related to nitric acid. This acid, as its chemical formula indicates, is formed by the combination of the three elements we have just studied, namely, hydrogen, nitrogen, and oxygen. Pure nitric acid is a colorless liquid. It gives off colorless, irritating fumes, when exposed to the air. Strong nitric acid acts violently upon many substances, particularly those of animal and vegetable origin, decomposing them very rapidly. Nitric acid burns the flesh, eats through clothing, disintegrates wood, and dissolves metals. It is one of the most active of chemical substances. The compounds of nitrogen that occur in food are very numerous and of complex composition. They will be discussed in Lessons III and IV, pages 99 and 125 respectively. CHLORIN [Sidenote: Sources of chlorin] Chlorin, though widely distributed in nature, does not occur in very large quantities as compared with oxygen and hydrogen. It is found chiefly in combination with the element sodium, as common salt or sodium chlorid, which is represented by the symbol NaCl. [Sidenote: Properties of chlorin] Chlorin is a greenish-yellow gas. It has a disagreeable smell and acts upon the passages of the throat and nose, causing irritation and inflammation. The feeling produced is much like that of a cold in the head. Inhaled in concentrated form, that is, not diluted with a great deal of air, it would cause death. It is much heavier than air, combines readily with other substances, and possesses the property of bleaching or destroying colors. HYDROCHLORIC ACID [Sidenote: Hydrogen and chlorin combined] Just as hydrogen burns in the air, so it burns in chlorin. The burning of hydrogen in air or oxygen is, as we have seen, simply the combination of hydrogen and oxygen, the product being water in the form of vapor, and therefore invisible. When hydrogen burns in chlorin, the action consists in the union of the two gases, the product being hydrochloric acid, HCl, which forms clouds in the air. The two gases, hydrogen and chlorin, may be mixed together and allowed to stand together indefinitely in the dark, and no action will take place. If, however, the mixture be put into a room lighted by the sun, but where the sun does not shine directly upon it, combination takes place gradually; but if the sun be allowed to shine directly upon the mixture for an instant, explosion occurs, this being the result of the combination of the two gases. The same result can be caused by applying a flame or spark to the mixture. In this case light causes chemical action. The art of photography depends upon the fact that light has the power to cause chemical changes. [Sidenote: Importance and preparation of hydrochloric acid]