90 SOURCE AND ASSIMILATION OF NITROGEN. 



tains in the gaseous form to fall as rain, then I'l 

 pound of rain water must be obtained from every 

 II5477 cubic feet of air. The whole quantity of am- 

 monia contained in the same number of cubic feet 

 will also be returned to the earth in this one pound 

 of rain-water. But if the 11,477 cubic feet of air 

 contain a single grain of ammonia, then ten cubic 

 inches, — the quantity usually employed in an analy- 

 sis, — must contain only 0.00QP00050 of a grain. 

 This extremely small proportion is absolutely inap- 

 preciable by the most delicate and best eudiometer ; ^ 

 it might be classed among the errors of observation, 

 even were its quantity ten thousand times greater. 

 But the detection of ammonia must be much more 

 easy when a pound of rain-water is examined, for 

 this contains all the gas that was diffused through 

 11,477 cubic feet of air. 



If a pound of rain-water contain only ^th of a grain 

 of ammonia, then a field of 26,910 square feet must 

 receive annually upwards of 88 lbs. of ammonia, or 

 71 lbs. of nitrogen ; for by the observations of Schu- 

 bler, which were formerly alluded to, about 770,000 

 lbs. of rain fall over this surface in four months, and 

 consequently the annual fall must be 2,310,000 lbs. 

 This is much more nitrogen than is contained in the 

 form of vegetable albumen and gluten, in 2920 lbs. 

 of wood, 3085 lbs. of hay, or 200 cwt. of beet-root, 

 which are the yearly produce of such a field ; but it 

 is less than the straw, roots, and grain of corn, w^hich 

 might grow on the same surface, would contain.f 



* A eudiometer is an instrument used in the analyses of the atmo- 

 sphere. It means a measure of purity. It is also used in the analysis 

 of mixtures of gases. Several varieties are described in Webster's 

 Manual^ p. 137. 



t The advocates of the importance of humus as a nourishment for 

 plants, being driven from their position by the facts brought forward in 

 the preceding chapters, have found in the ammonia of the atmosphere 

 an explanation of the manner in which humus acquires its solubility, 

 and therefore its capability of being assimilated by plants. Now, it is 

 very true that humic acid is soluble in ammonia ; but the humic acid 

 of chemists is not contained in soils. Were it so, on treating mould with 

 water we should obtain a dark-colored solution of humate of ammonia. 

 But we obtain a solution which is entirely devoid of this acid. It cau- 



