426 Action of the AtmospJiere upon newly-deepened Soil. 



nitrogen must take place. Liebig however, though attributing 

 considerable importance to the ammonia of the air as a source 

 of nitrogen, considers that the nitric acid furnished to the earth 

 in Europe by rains is so extremely small in amount that its 

 influence cannot be considered as a source of nitrogen to plants. 

 Chevallier noticed in the air at Paris ammonia and organic 

 matters, and in London sulphurous acid. Boussingault and 

 others add a small proportion of carburetted hydrogen. 



The following constitution has been assigned to 10,000 volumes 

 of dry air : — 



Nitrogen 7,914 



Oxygen 2,082 



Carbonic acid 4 



Carburetted hydrogen . . . . a trace. 



Ammonia a trace. 



10,000 



Sulphuretted hydrogen and phosphuretted hydrogen are also 

 pretty generally present, but in infinitesimal proportions, appa- 

 rently derived from the decomposition of animal and vegetable 

 matter. 



Dr. Witting, of Hoxter on the Weser, remarked that the 

 atmosphere of a place contained in general the same foreign 

 ingredients which the first fall of rain brings to the ground ; 

 such, for example, as traces of muriates, of free muriatic and 

 carbonic acids, and of carburetted hydrogen gas. Rain which 

 fell during a N.VV. wind commonly contained much carbonic, 

 together with traces of phosphoric, acid. Tlie latter was dis- 

 covered on several occasions in rain which had fallen during 

 particular states of the weather ; and Dr. Witting also states that 

 certain plants exhale it, so that, when they are confined under 

 glass, traces of this acid may be detected on the internal surface 

 of the latter. (Quoted by Daubeny, Brit. Assoc. Report, 1836, 

 p. 2.) 



With reference to the above, it might be interesting to inquire 

 to what extent the soil in the neighbourhood of large towns is 

 enriched by the many ingredients that are found in the air over 

 them. Thus, Dr. R. A. Smith, of Manchester, in his papers on 

 the air and water of towns, speaking of rain, says, " Collected in 

 a town, we know it to be a nauseous and black liquid ; and wlien 

 we go a mile from a town it is no less nauseous, althoagli 

 it loses its blackness. This would show that the black soot from 

 chimneys is deposited very near a town, although the soluble 

 substances are carried farther. Even many miles round a town 

 the rain is unfit for use, without being passed through purifying 

 materials. I have tried it as far as ten miles from Manchester; 

 and it is probable that it is nowhere free from objection, as it 



