40 FAMILIAR LETTERS ON CHEMISTRY. 



moist, but becoming in the dry air a powder. This is the phosphate of lime of the 

 bones, with scarcely one per cent, of foreign organic matter. 



Thus we see that in the solid and fluid excrements of man and animals, all the 

 nitrogen in short, all the constituent ingredients of the consumed food, soluble and 

 insoluble, are returned ; and as food is primarily derived from the fields, we possess 

 in those excrements all the ingredients which we have taken from it in the form of 

 seeds, roots, or herbs. 



One part of the crops employed for fattening sheep and cattle is consumed by 

 man as animal food ; another part is taken directly as flour, potatoes, green vege- 

 tables, &c. ; a third portion consists of vegetable refuse, and straw employed as 

 litter. We can, it is obvious, get back all its constituent parts which have been 

 withdrawn therefrom, as fruits, grain, and animals, in the fluid and solid excre- 

 ments of man, and the bones, blood, and skins of the slaughtered animals. It 

 depends upon ourselves to collect carefully all these scattered elements, and to 

 restore the disturbed equilibrium of composition in the soil. We can calculate 

 exactly how much and which of the component parts of the soil we export in a 

 sheep or an ox, in a quarter of barley, wheat, or potatoes, and ^ve can discover, from 

 the known composition of the excrements of man and animals, how much we have 

 to supply to restore what is lost to our fields. 



If, however, we could procure from other sources, the substances which give to 

 the exuviae of man and animals their value in agriculture, we should not need the 

 latter. It is quite indifferent for our purpose whether we supply the ammonia (the 

 source of nitrogen) in the form of urine, or in that of a salt derived from coal-tar ; 

 whether we derive the phosphate of lime from bones, apatite, or fossil excrements 

 (the coprolithes.) 



The principal problem for agriculture is, how to replace those substances which 

 have been taken from the soil, and which cannot be furnished by the atmosphere. 

 If the manure supplies an imperfect compensation for this loss, the fertility of a 

 field, or of a country, decreases ; if, on the contrary, more are given to the fields, 

 their fertility increases. 



An importation of urine, or of solid excrements, from a foreign country, is equiva- 

 lent to an importation of grain and cattle. In a certain time, the elements of those 

 substances assume the form of grain, or of fodder, then become flesh and bones, 

 enter into the human body, and return again day by day to the form they originally 



The only real loss of elements we are unable to prevent is of the phosphates, 

 and these, in accordance with the customs of all modern nations, are deposited in 

 the grave. For the rest, every part of that enormous quantity of food which a man 

 consumes during his lifetime (say in sixty or seventy years,} which was derived 

 from the fields, can be obtained and returned to them. We know, with absolute 

 certainty, that in the blood of a young or growing animal there remains a certain 

 quantity of phosphate of lime and of the alkaline phosphates, to be stored up and 

 minister to the growth of the bones and general bulk of the body, and that, with 

 the exception of this very small quantity, we receive back, in the solid and fluid 

 excrements, all the salts and alkaline bases, all the phosphate of lime and mag- 

 nesia, and consequently all the inorganic elements which the animal consumes in 

 its food. 



We can thus ascertain precisely the quantity, quality, and composition of animal 

 excrements, without the trouble of analysing them. If we give a horse daily 4 

 pounds' weight of oats, and fifteen pounds of hay, and knowing that oats give four 

 per cent, and hay nine per cent, of ashes, we can calculate that the daily excre- 

 ments of the horse will contain twenty-one ounces of inorganic matter which was 

 drawn from the fields. By analysis we can determine the exact relative amount 

 of silica, of phosphates, and of alkalies, contained in the ashes of the oats and of 

 the hay. 



You will now understand that the constituents of the solid parts of animal 

 excrements, and therefore their qualities as manure, must vary with the nature of 

 the creature's food. If we feed a cow upon beetroot, or potatoes, without hay, 

 straw, or grain, there will be no silica in her solid excrements, but there will be 

 phosphate of lime and magnesia. Her fluid excrements will contain carbonate of 

 potash and soda, together with compounds of the same bases with inorganic acids. 

 In one word, we have in the fluid excrements, all the soluble parts of the ashes of 

 the consumed food ; and in the solid excrements, all those parts of the ashes which 

 are insoluble in water. 



If the food, after burning, leaves behind ashes containing soluble alkaline phos- 

 phates, as is the case with bread, seeds of all kinds, and flesh, we obtain from the 

 animal by which they are consumed a urine holding in solution these phosphates. If, 



