MATERIAL BESOUBCES OF LIFE. 341 



over nine weights in a hundred of the body of man. It is obtained chiefl}" 

 by the plants; mostly from water, but to some extent from ammonia, the 

 the latter being more notable as a source of another element. 



Water is not quite always as free as air — failing the needs of the sta- 

 tionary bodies of plants more often than it does the wants of animals, and 

 in the quantities taken as food by man hardly liable to a notable value in 

 exchange. Asa substance not wholly gaseous, it is not easy to conceive 

 how water could be more abundantly supplied than it is, without being a 

 burden and a hindrancefto life. It is doubtful whether mankind would 

 vote for any uniform increase in the quantit}' of water on the planet. If 

 water was supplied in vapor more abundantl}^ than it is, by having a lower 

 vaporizing point, the conditions of all life would be changed — the atmos- 

 phere would be put out of its adjustment with the organic creations. 



Some of the simpler forms of life subsist almost wholly upon the three 

 elemental materials we have had in consideration, with a few others of the 

 plentiful resources; and living beings taken together use much larger 

 quantities of these than of the substances more sparingly supplied. But, 

 as to the relative importance of the tAvo classes of resources, it can onl}^ be 

 said that the higher forms of life can no more exist without the one than 

 without the other. 



Of the adequate resources, nitrogen is needed by the largest number of 

 living bodies and used in the largest quantities. It enters into most animal 

 tissues and the more complex of the vegetable products; being two and a 

 half parts in a hundred of the body of man, or eight per cent, of all its sol- 

 ids. It is obtained for the organic world solely by the plants, and obtained 

 only from combinations of nitrogen, the ammonia and nitrates of the air 

 and the soil. 



The supply of this combined or available nitrogen in the air is limited 

 — enough for a measure of vegetation, but not near enough for the greatest 

 growth of food-plants and grains. The quantity of combined nitrogen car- 

 ried by the rain from the air to the plant roots was found to be, in the rain- 

 fall of a year in G-reat Britain, equal to seven pounds of ammonia on an 

 acre; another year it equaled nine and a half pounds per acre. The con- 

 stituents of wheat are such that twenty-four bushels require the nitrogen 

 of forty-five pounds of ammonia; that is, for the crop on a given surface, 

 about five times as much as the rain furnishes. Plants doubtless gather 

 directly from the nitrogen compounds of the air without help of the rain, 

 and obtain a larger supply from the organic mould of good soils; but that 

 all these sources together provide hardly enough is pretty clearly proved 

 by feeding the roots of the plants with additional nitrogen compounds. On 

 all but the richest soils, the suitable application of ammonia or nitrates 

 causes a notable increase in the quantit}^ of food-plants, and also causes an 

 increased proportion of the nitrogenous constituents of plants. If nitrogen 

 compounds could be laid down cheaply enough, it would augment the sup- 

 plies of food and raiment, and the comfort of man, in no small degree. 



