188 On the Function of Salt in Agriculture. 



although from its partial precipitation by carbonate of ammonia 

 I should almost think that it existed in the latter form ; since 

 it is more rational to suppose that carbonate of ammonia, being 

 a less permanent combination than carbonate of soda, should 

 transfer its carbonic acid to chloride of calcium, taking hydro- 

 chloric acid in return, than that it should exert any action upon 

 carbonate of lime dissolved as such. The idea of the consider- 

 able temporary solubility of recently formed carbonate of lime, 

 receives considerable support from a peculiar pha;nomenon which 

 took place when I endeavoured to make carbonic acid determi- 

 nations in the Worcestershire brine-springs. I adopted the ordi- 

 nary process of adding to the water excess of ammoniacal chlo- 

 ride of calcium, — a very trifling precipitate occurred, which was 

 attributed to the comparative absence of carbonates and carbonic 

 acid ; upon examining the bottles at home, however, their interior 

 was covered with most beautiful crystals of selenite, of the most 

 perfect forms, — some complete hemitropes. From their figure and 

 size, these had evidently been deposited slowly; and I do not doubt 

 now that the chloride of sodium at first prevented the precipi- 

 tation of the carbonate of lime, then that the large quantity of 

 sulphate of soda present parted with its sulphuric acid to the 

 lime so dissolved (by degrees, as the chloride of sodium relaxed 

 its solvent power) ; the liquid was soon saturated with sulphate 

 of lime, and the surplus was thus gradually deposited. I need 

 not say that the amount of carbonate of lime actually thrown 

 down was quite insufficient to meet the requirements of the ana- 

 lysis ; nor is this the only instance in which I have observed a 

 similar result. 



The results, then, at which we must arrive are, that agricul- 

 tural salt is a most energetic absorbent of ammonia, both in 

 virtue of its chloride of sodium and of its soluble lime-salt, and 

 that the proportion of the latter especially most powerfully affects 

 its action ; — but that at the same time its agency does not seem 

 to be altogether a permanent one : it will collect the ammonia, but 

 it is questionable whether it can retain it for any great length of 

 time, because in the very decompositions which happen in order 

 to render the ammonia more stable, salts are formed which have 

 a direct tendency to liberate ammonia from its more fixed com- 

 binations. It may, however, retain it quite long enough for 

 agricultural purposes : if the young plants are there ready to 

 receive it, its state of gradual liberation may be for them the most 

 advantageous possible ; and to this conclusion all experiments on 

 the large scale appear most obviously to tend. It is described as 

 an excellent check to the too forcing power of guano ; and from 

 M. BarraFs experiment we see that it either prevents the too 

 rapid eremacausis of the latter, or stores up the ammonia as it is 



I 



