THE MATERIAL RESOURCES OF LIFE. 343 



rate contrivances to exclude the cold and wet and wind and glare of 

 the weather, can make but slight impediments to its distribution. 



One other element we were to inquire of, among the redundant 

 materials : the unit of chemical measures, hydrogen. As light as it 

 is, it makes over nine weights in a hundred of the body of man. It is 

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

 from ammonia, 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 

 stationary bodies of plants more often than it does the wants of ani- 

 mals, and in the quantities taken as food by man hardly liable to a 

 notable value in exchange. As a 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 hinderance to life. It is doubt- 

 ful whether mankind would vote for any uniform increase in the quan- 

 tity of water on the planet. If water was supplied in vapor more 

 abundantly than it is, by having a lower vaporizing-point, the con- 

 ditions of all life would be changed the atmosphere 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 spar- 

 ingly supplied. But, as to the relative importance of the two classes 

 of resources, it can only 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 num- 

 ber 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 its solids. It is obtained for the organic world solely by 

 the plants, and obtained only from combinations of nitrogen, the am- 

 monia 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 com- 

 bined nitrogen carried by the rain from the air to the plant-roots was 

 found to be, in the rainfall of a year in Great Britain, equal to seven 

 pounds of ammonia on an acre ; another year it equaled nine and 

 a half pounds per acre. The constituents of wheat are such that 

 twenty-four bushels require the nitrogen of forty-five pounds of am- 

 monia ; that is, for the crop on a given surface, about five times as 

 much as the rain furnishes. Plants dovibtless 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 



