We have learned that the air is composed chiefly of oxygen and nitrogen These are not combined as oxygen and hydrogen are in water, but are simply mixed together, four-fifths of the mixture being nitrogen. Nitrogen is also found in combination in a large number of substances in nature. It is found in the nitrates, as saltpeter or potassium nitrate, KNO3, and Chili saltpeter or sodium nitrate, NaNO3. It is also found in the form of ammonia, which is a compound of nitrogen and hydrogen of the formula NH3, and exists in that form in a limited quantity of the air. In most foods, especially in those of animal origin, nitrogen occurs in chemical combination.

Sources of nitrogen.

Nitrogen is a colorless, tasteless, odorless gas which does not burn, and does not combine readily with oxygen, or with any other element except at a very high temperature, and except in the formation of living plants, or in animal life. Just as nitrogen does not support combustion, so also it does not support life. An animal would die confined in a tank of nitrogen, not on account of any active poisonous properties in the nitrogen, but for lack of oxygen.

When a compound containing carbon, hydrogen and nitrogen is heated in a closed vessel, so that the air is excluded, and so that it cannot burn, the nitrogen passes out of the compound, not as nitrogen, but in combination with hydrogen, which forms ammonia. Nearly all animal substances contain carbon, hydrogen, oxygen, and nitrogen, and many of them give off ammonia when heated as above described.

Properties of nitrogen.

Compounds of nitrogen.

Ammonia is written by the chemist NH3, or one part of nitrogen gas to three parts of hydrogen. It is a colorless, transparent gas with a very penetrating, characteristic odor. In concentrated form it causes suffocation. It is but little more than half as heavy as air. It is easily converted into liquid form by pressure and cold. When pressure is removed from the liquefied ammonia, it passes back very rapidly into gaseous form, and in so doing it absorbs heat. Investigators have taken advantage of these facts and are employing liquid ammonia in the manufacture of artificial ice.

While air is merely a mixture of oxygen and nitrogen, this does not prove that these two elements cannot unite. In fact they do unite in five different proportions so as to form five different substances.

Why ammonia is used in making artificial ice.

These are given below to illustrate how different substances can be formed from the same things, by merely combining them in different proportions. This example is also given to impress upon the mind of the practitioner the great importance of proportioning nutritive elements in diet so that the patient will not be overfed on some elements while underfed on others. It is absolutely essential, in order to know what effect a substance will have in the laboratory, or in the body, to know not only of what it is composed, but with what substances and in what proportions it is combined.

Nitrous oxid .....N20

Nitric oxid ......NO or N202

Nitrogen trioxid . . N203 Nitrogen peroxid . . N02 or N204

Nitrogen pentoxid N205

To further illustrate the wonders of chemical combinations, we give the properties of two of these oxygen and nitrogen compounds:

Importance of proportioning food.

Nitrous oxid, N20, is colorless, transparent, and has a slightly sweetish taste. When inhaled it causes a kind of intoxication which manifests itself in the form of hysterical laughing, hence it is commonly called "laughing gas." Inhaled in larger quantities it causes unconsciousness and insensibility to pain. It is, therefore, used in many surgical operations, particularly by dentists in extracting teeth. Nitrogen peroxid, NO2, is a reddish-brown gas. It has an extremely disagreeable odor and is very poisonous.

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.

Properties and uses of nitrous oxid.

Composition of 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.