460 
absolute contact with the glass the force with which the 
liquid would penetrate between them would be very great. 
It therefore appears to me nearly certain that the pressure 
would be to a considerable extent sustained by a thin film of 
liquid, which being thus under pressure would dissolve more 
salt than the rest of the solution, and, by slow diffusion 
amongst it, the salt would thus be transferred from where the 
pressure is greatest to where it is less. 
Now these experimental results entirely satisfy the con- 
ditions met with in the case of the impressed limestone 
pebbles. Pressed one against the other with great force, at 
a considerable depth below the surface of the earth, and sur- 
rounded with water saturated with carbonate of lime, in 
accordance with the principles I have described the limestone 
would dissolve, so that in time one pebble would penetrate 
into the other, and carbonate of lime would be deposited in a 
crystalline form elsewhere, where the pressure was less. This 
explanation agrees admirably with the various facts. The 
structure of the limestone proves most conclusively that the 
depressions were produced by the actual removal of material, 
and not by its yielding as a plastic substance. Moreover, it is 
only the carbonate of lime which has been removed — only the 
soluble part of the pebble — the insoluble earthy portion 
having been left behind at the bottom of the depressions ; 
and therefore the removal cannot have been effected by mere 
mechanical means, which would have removed the whole 
indiscriminately. I attribute the solution of the material of 
one of the pebbles, and the unaltered outline of the other, to 
a difference in their hardness, or the amount of earthy or 
sandy impurities; whilst, at the same time, I think it proba- 
ble that a difference in curvature may have considerable 
influence. 
If then the principle of the convertibility of mechanical 
pressure into chemical action serves so well to explain the 
