8. Hydrogen

This element, which, in a free state, is the lightest known gas, is found abundantly in nature only in combination with other elements. The minute quantities which exist in the air are due to volcanic action and possibly to decay under certain conditions. As a manufactured product, it has an important use in producing intense heat and in filling balloons. Hydrogen constitutes about one-ninth of water by weight, and is found in a large number of soil compounds. It is an essential constituent of vegetable and animal tissues, although it exists in the compounds of living organisms in a much smaller proportion than carbon or oxygen. Plants obtain it largely from water, and it is furnished to the animal body in water and in other compounds.

9. Nitrogen

Probably no element has been given more attention in its relations to human nutrition than has nitrogen as such and in its compounds. Like oxygen, it is an invisible, tasteless, and odorless gas which forms in the free state a large part of the earth's atmosphere. The air has been considered to be approximately 77 per cent free nitrogen by weight, but the discovery of the new element, argon, which has heretofore passed as nitrogen, will slightly modify previous determinations.

Nowhere outside of the air, in the tissues of living organisms, and in certain mineral deposits, does nitrogen exist in any form in comparatively large quantities. The soil spaces contain it, and it is taken into solution in small proportions by all natural waters. It is found in the mineral, as well as organic, compounds of the soil, but in quantities which seem insignificant as compared with other elements, such as oxygen and silicon. Few agricultural soils contain over one-half of one per cent of combined nitrogen. Minute quantities of its compounds exist in the atmosphere, which are being constantly carried to the soil in rain-water and as constantly replaced by the ammonia from decomposing animal and vegetable matter and by the products of the oxidation of nitrogen through electrical action and combustion. Notwithstanding this comparatively small supply of nitrogen compounds, they play a prominent part in human nutrition both commercially and physiologically.

Nitrogen compounds are especially important because the available supply is often dangerously near the demand, or even below it. The nitrogen found in the air is inert for animal uses, and is ignored by a large majority of plants. Much of that in the soil is also unavailable. Moreover, its immediately useful compounds on the farm are constantly subject to loss, - first by processes of fermentation, such as those in manures, which the farmer cannot wholly prevent, and second by soil losses which are to some extent beyond control. Many of the commercial products of the farm also carry away much nitrogen.. The sources of supply to balance this outgo are the nitric acid and ammonia of the rainfall, the free nitrogen captured by legumes, and whatever comes from purchased fertilizers and foods. These facts relate primarily to plant production, but they also sustain an essential relation to the maintenance of human life, and cannot be ignored in a discussion of the physical problems that relate to human welfare.

Physiologically, the nitrogen compounds stand in the front rank. They are necessary building material for the fundamental tissues of the animal, and are intimately related to the prominent chemical changes that are involved in growth and in the maintenance of life. It is safe to assert, too, that variations of these compounds in the food may have an important influence on health and the character of the body structure.

As a result of these conditions which relate to the supply of useful nitrogen and to its important r61e, we find that it has assumed a prominent place in commerce. For these reasons, the control, even though only partial, which the farmer may now assume over the income and outgo of the nitrogen compounds valuable to agriculture is a triumph of modern science and an important feature of national economics.