218 BULLETIN OF THE BUSSEY INSTITUTION, 
On the other hand, it is not unreasonable to expect, that in the fact 
of the formation, and in the reactions of supercarbonate of lime in dry 
soils, may be found the true explanation of the use of lime as a fertil- 
izer upon light soils, as in the agriculture of some parts of France, and 
of the use of iéached ashes upon our own dry soils that overlie the 
sand and gravel of the drift. As Professor Johnson, of New Haven, 
has recently insisted anew,* leached ashes are really a calcareous 
manure; and the fact of their utility is unquestionably connected in 
some way with the carbonate of lime they contain. But if it can be 
shown that supercarbonate of lime is formed with special abundance in 
dry soils when these fertilizers are used upon them, and that, as would 
seem to be proved by the percolation experiments recorded in this article, 
this supercarbonate moves about freely in the earth on the arrival of 
moisture, we would have gained no small item of evidence to explain 
the mode of action of the lime and the ashes; for there can be no 
question that the supercarbonate of lime would exert useful actions, 
both by setting free potash from its compounds in the earth, and by 
promoting the decomposition of organic matters in the soil; for, as is 
well known, the solution of supercarbonate of lime is a somewhat — 
alkaline liquid. 
The power of solutions of supercarbonate of lime to decompose 
minerals has often been shown by direct experiments; f and there are 
numerous observations of facts upon record that appear to have de- 
pended in reality upon this power of the supercarbonate, rather than 
upon the action of simple carbonic-acid water, to which they were 
referred. 
The speedy formation of solutions of supercarbonate of lime, as 
shown by the preceding experiments, when dry soils are moistened, 
goes to show that the action of mere carbonic acid must be limited 
to the moment when the gas, or the water that contains it, first comes 
in contact with the soil. Subsequently, the action will be due not to 
carbonic-acid water, but to a solution of supercarbonate of lime. 
Geologists seem often to have attached undue importance to the dis- 
integrating action of carbonic acid brought directly from the air by 
rain and the other forms of precipitated moisture. It is true, of 
* See this Bulletin, 1. 202. 
+t Compare, Bischof’s ‘‘ Chemical and Physical Geology,” London, 1859, 3. 
pp. 90, 91; and Fittbogen, in Hoffmann’s “ Jahresbericht,” 1873-74, 1, 7. 
