45 FAMILIAR LETTERS ON CHEMISTRY. 



is a most abundant supply. The ammonia contained in animal excrements exer- 

 cises a favorable effect, inasmuch as it is accompanied by the other substances 

 necessary to accomplish its transition into the elements of blood. If we supply am- 

 monia associated with all the conditions necessary to its assimilation, it ministers 

 to the nourishment of the? plants ; but if this artificial supply is not given, they can 

 derive all the needed nitrogen from the atmosphere a source, every loss from which 

 is restored by the decomposition of the bodies of dead animals and the decay of 

 plants. Ammonia certainly favors and accelerates the growth of plants in all soils, 

 wherein all the conditions of its assimilation are united ; but it is altogether without 

 effect, as J respects the production of the elements of blood where any of these con- 

 ditions are wanting. We can suppose that asparagin, the active constituent of 

 asparagus, the mucilaginous root -o^ the marsh-mallow, the nitrogenised and sul- 

 phurous ingredients of mustard-seed, and of all cruciferous plants, may originate with- 

 out the aid of the mineral elements of the soil. But if the principles of those 

 vegetables, which serve as food, could be generated without the co-operation of the 

 mineral elements of blood, without potash, soda, phosphate of soda, phosphate of 

 lime, they would be useless to us and to herbivorous animals as food ; they would 

 not fulfil the purpose for which the wisdom of the Creator has destined them. In 

 the absence of alkalies and the phosphates, no blood, no milk, no muscular fibre, 

 can be formed. Without phosphate of lime, our horses, sheep, and cattle, would be 

 without bones. 



In the urine and in the solid excrements of animals we carry ammonia, and con- 

 sequently nitrogen, to our cultivated plants, and this nitrogen is accompanied by 

 all the mineral elements of food exactly in the same proportions in which both are 

 contained in the plants which served as food to the animals, or, what is the same, 

 in those proportions in which both can serve as nourishment to a new generation of 

 plants, to which both are essential. 



The effect of an artificial supply of ammonia, as a source of nitrogen, is, there- 

 fore, precisely analogous to that of humus as a source of carbonic acid it is 

 limited to a gain of time ; that is, it accelerates the development of plants. This is 

 of great importance, and should always be taken into account in gardening, espe- 

 cially in the treatment of the kitchen garden ; and as much as possible in agriculture 

 on a large scale, where the time occupied in the growth of the plants cultivated is 

 of importance. 



When we have exactly ascertained the quantity of ashes left after the combustion 

 of cultivated plants which have grown upon all varieties of soil, and have obtained 

 correct analysis of these ashes, we shall learn with certainty which of the consti- 

 tuent elements of the plants are constant and which are changeable, and we shall 

 arrive at an exact knowledge of the sum of all the ingredients we withdraw from 

 the soil in the different crops. 



With this knowledge the farmer will be able to keep an exact record of the pro- 

 duce of his fields in harvest, like the account-book of a well regulated manufactory ; 

 and then, by a simple calculation, he can determine precisely the substances he must 

 supply to each field, and the quantity of these, in order to restore their fertility. 

 He will be able to express, in pounds weight, how much of this or that element he 

 must give to the soil in order to augment its fertility for any given kind of plants. 



These researches and experiments are the great desideratum of the present time. 

 To the united efforts of the chemists of all countries we may confidently look for a 

 solution of these great questions, and by the aid of enlightened agriculturists we 

 shall arrive at a rational system of gardening, horticulture, and agriculture, appli- 

 cable to every country and all kinds of soil, and which will be based upon the 

 immutable foundation of observed facts and philosophical inductions. 



LETTER XVI 



MY DEAR SIR : 



My recent .researches into the constituent ingredients of our cultivated fields 

 have led me to the conclusion that, of all the elements furnished to plants by the soil 

 and ministering to their nourishment, the phosphate of lime, or, rather, the phos- 

 phates generally, must be regarded as the most important. 



In order to furnish you with a clear idea of the importance of the phosphates, it 



