FAMILIAR LETTERS ON CHEMISTR 



sphere, in whatever state it may be when absorbed, from the atmosphere it must 

 have been derived. Did not the fields of Virginia receive their nitrogen from the 

 same source as wild plants ? 



Is the supply of nitrogen in the excrements of animals quite a matter of indiffer- 

 ence, or do we receive back from our fields a quantity of the elements of blood 

 corresponding to this supply? 



The researches of Boussingault have solved this problem in the most satisfactory 

 manner. If, in his grand experiments, the manure which he gave to his fields was 

 in the same state, that is, dried at 110 in a vacuum, as it was when analysed, these 

 fields received, in sixteen years, thirteen hundred pounds of nitrogen. ' But we 

 know that by drying all the nitrogen escapes which is contained in solid animal 

 excrements, as volatile carbonate of ammonia. In this calculation the nitrogen of 

 the urine, which by decomposition is converted into carbonate of ammonia, has not 

 been included. If we suppose it amounted to half as much as^ that in the dried 

 excrements, this would make the quantity of nitrogen supplied to the fields one 

 thousand nine hundred and fifty pounds. 



In sixteen years, however, as we have seen, only one thousand five hundred and 

 seventeen pounds of nitrogen was contained in their produce of grain, straw, roots, 

 etcetera that is, far less than was supplied in the manure; and in the same 

 period, the same extent of surface of good meadow-land (one hectare=a Hessian 

 morgen,) which received no nitrogen in manure, two thousand and sixty pounds of 

 nitrogen. 



It is well known that in Egypt, from the deficiency of wood, the excrement of 

 animals is dried, and forms the principal fuel, and that the nitrogen from the soot 

 of this excrement was, for many centuries, imported into Europe in the form of sal 

 ammoniac, until a method of manufacturing this substance was discovered at the 

 end of the last century by Gravenhorst of Brunswick. The fields in the Delta of the 

 Nile are supplied with no otber animal manure than the ashes of the burnt excre- 

 ments, and yet they have been proverbially fertile from a period earlier than the 

 first dawn of history, and that fertility continues to the present day as admirable 

 as it was in the earliest times. These fields receive, every year, fronr the inunda- 

 tion of the Nile, a new soil, in its mud deposited over their surface, rich in those 

 mineral elements which have been withdrawn by the crops of the previous harvest. 

 The mud of the Nile contains as little nitrogen, as the mud derived from the Alps 

 of Switzerland, which fertilises our fields after the inundations of the Rhine. If 

 this fertilising mud owed this property to nitrogenised matters; what enormous 

 beds of animal and vegetable exuviae and remains ought to exist in the mountains 

 of Africa, in heights extending beyond the limits of perpetual snow, where no bird, 

 no animal finds food, from the absence of all vegetation ? 



Abundant evidence in support of the important truth we are discussing, may be 

 derived from oth<y well known facts. Thus, the trade of Holland in cheese may 

 be adduced in proof and illustration thereof. We know that cheese is derived from 

 the plants which serve as food for cows. The meadow-lands of Holland derive the 

 nitrogen of cheese from the same source as with us ; that is, the atmosphere. The 

 milch cows of Holland remain day and night on the grazing-grounds, and therefore, 

 in their fluid and solid excrements return directly to the soil all the salts and earthy 

 elements of their food : a very insignificant quantity only is exported in the cheese. 

 The fertility of these meadows can, therefore, be as little impaired as our own 

 fields, to which we restore all the elements of the soil, as manure, which have been 

 withdrawn in the crops. The only difference is, in Holland they remain on the 

 field, while we collect them at home, and carry them, from time to time to the fields. 



The nitrogen of the fluid and solid excrements of cows, is derived from the 

 meadow-plants, which receive it from the atmosphere ; the nitrogen of the cheese 

 also must be drawn from the same source. The meadows of Holland have, in the 

 lapse of centuries, produced millions of hundred weights of cheese. Thousands of 

 hundred weights are annually exported, and yet the productiveness of the meadows 

 is in no way diminished, although they never receive more nitrogen than they origi- 

 nally contained. 



Nothing, then, can be more certain than the fact, that an exportation of nitrogenised 

 products does not exhaust the fertility of a country ; inasmuch as it is not the soil, 

 but the atmosphere, which furnishes its vegetation with nitrogen. It follows, con- 

 sequently, that we cannot increase the fertility of our fields by a supply of nitro- 

 genised manure, or by salts of ammonia, but rather that their produce increases or 

 diminishes in a direct ratio with the supply of mineral elements capable of assimi- 

 lation. The formation of the constituent elements of blood that is, of the nitro- 

 genised principles in our cultivated plants depends upon the presence of inorganic 

 matters in the soil, without which no nitrogen can be assimilated, even when there 



