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proved that in the same manner that we may obtain me- 
chanical force by means of chemical changes, so we may 
modify or produce chemical changes by mechanical pressure. 
All previous writers had attempted to explain the im- 
pressions in the pebbles either by mere mechanical or simple 
chemical action, localized by mechanical conditions. The 
facts point so strongly to both agencies, that it is easy to 
understand why the opinions ever varied between the two 
extremes; and it now appears to me astonishing that no one 
was led at an earlier period to suggest a correlated action of 
both. At the same time we must admit that a large part of 
those who have studied the question, did so before the 
doctrines of the correlation and conservation of force were 
generally admitted or understood. It is mainly as an illus- 
tration of the applicability of such principles to geology 
that I have been led to draw attention to the impressed 
pebbles. 
In my Paper already cited, I showed that in the case of 
the majority of substances mechanical pressure increases 
their solubility. For example, if a crystal of common salt is 
placed in a perfectly concentrated solution, so that no more 
would be dissolved under the ordinary pressure, on applying 
to the solution a pressure of, for example, a thousand pounds 
to the square inch, and maintaining it for a sufficiently long 
time, more salt is dissolved, and is again deposited on re- 
moving the pressure. I also found that a glass rod with a 
small round termination, pressed with a force of ten pounds 
against a plate of rock salt, had in the course of a year pro- 
duced a well-marked depression, surrounded by a ring of 
small crystals raised above the level of the original surface. 
The salt had been exposed to the atmosphere, in which case 
a thin film of moisture is generally present on the surface; 
and, since the force of capillary attraction varies inversely as 
the width between two plates, where the salt was in almost 
