334 AI\IMO^^IACAL SALTS. 



It is then very probable that if the ammoniacal salts afford azote 

 to the plants, they enter not as muriate, sulphate, or phosphate, be- 

 cause there is no reason to believe that acids united with an alkali 

 are eliminated almost in totality during the act of vegetation. It 

 necessarily follows, that the ammonia of these salts, in order to yield 

 to vegetables its constituent azote, must reach their organs in the 

 form of carbonate, inasmuch as that is the sole ammoniacal salt 

 uhich seems to exercise a direct and favorable agency. 



However, if such be the case, how comes it to pass, that ammo- 

 niacal salts, as the muriate, phosphate, and sulphate, are converted 

 into carbonate when once incorporated in the soil 1 Good arable 

 land almost always contains, it is true, carbonate of lime ; but there 

 is no ground for admitting an acid interchange betwixt the calca- 

 reous and ammoniacal salts. We know, on the contrary, that car- 

 bonate of ammonia reacts instantaneously upon muriate and sulphate 

 of lime, the products of this reaction being on the one hand muriate 

 and sulphate of ammonia, on the other, carbonate of lime. The gvp- 

 sum theory of Liebig is based upon the fact of this double decompo- 

 sition, whereby the ammoniacal carbonate of rain-water is fixed in 

 the state of sulphate at the cost of the sulphate of lime put on the 

 ground as manure. 



This reaction of sulphate of lime upon carbonate of ammonia is 

 incontestable as a laboratory experiment ; but in well-lilled grounds 

 containing* just the proper quantity of moisture, the reaction takes 

 place in the inverse sense. The carbonate of lime reacts upon the 

 sulphate of ammonia, and there result carbonate of ammonia and 

 sulphate of lime. 



This fact, however singular at first sight, is explained upon prin- 

 ciples established l)y Berthollet in liis chemical statics : 



When two saline solutions are mixed together, and from the mix- 

 ture an insoluble salt results, the insoluble comjxjund is formed and 

 precipitated. This is what happens on pouring a solution of car- 

 oonate of ammonia into one of sulphate of lime. But if, instead of 

 oringing the two salts together dissolved, they are mixed in a pul- 

 verulent state, and just sufficient water is added to promote the reac- 

 tion without dissolving the products, a volatile compound forms and 

 is evolved — namely, carbonate of ammonia. 



The experimental proof is easy, and not without interest. If chalk 

 previously washed be intimately mixed with crystallized sulphate of 

 ammonia, no change ensues, provided the powders are very dry. 

 Let moist sand be introduced so as to impart to the mixture the con- 

 sistence of light arable land of the usual humidity ; at the very in- 

 stant vapors of carbonate of ammonia, cognizable by their action on 

 vegetable colors and their odor, are developed. When water is 

 added in excess, the disengagement of ammoniacal vapor immedi- 

 ately ceases. The carbonate of ammonia not yet volatilized, dis- 

 solves and acts upon the ready formed gypsum so as to constitute 

 sulphate of ammonia and carbonate of lime. The ordinary reaction 

 is restored. Finally, this watery mixture being exposed to the air 

 furnishes anew ammoniacal vapors in proportion as the water yapor 



