FREEZING OF ICE. 
151 
the inside of tea-kettles, steamboat boilers, &c. ; 
also to the abundance of silex found in some springs 
possessing petrifying properties. 
Dr. Turner, of London, has shown, that in the de- 
composition of rocks containing feldspar, producing 
the well-known substance called porcelain clay, the 
quantity of silex carried off by solution was enor- 
mous. He attributes this loss to the freedom with 
which it could be dissolved when exposed to the 
united action of water and alkali at the moment of 
passing from the state of combination which con- 
stitutes feldspar.* This loss of silex in many of the 
rocks would in a great measure destroy their co- 
hesion, and powerfully tend to their rapid decompo- 
sition. 
III. The Freezing and Thawing of Ice, 
It is a matter of common observation that rocks 
are constantly undergoing changes in the Northern 
latitudes, especially from the freezing and thawing 
of ice. The amazing force exerted by freezing 
water is not generally known : that it is immense 
will appear from the following facts : 
Mr. Boyle filled a brass tube, three inches in di- 
ameter, with water, and confined it by means of a 
moveable plug ; the expansion, when it froze, took 
place with such violence as to push out the plug, 
though preserved in its situation by a weight equal 
to 70 pounds. The Florentine academicians burst 
a hollow brass globe, whose cavity was only an 
inch in diameter, by freezing the water with which 
it was filled ; and it has been estimated that the ex- 
pansive power necessary to produce such an effect 
was equal to a pressure of 27,720 pounds' weight. 
Bombs and cannon were burst in the same manner 
at Quebec in the years 1784 and 1785.t 
* DelaBeche. ' 
t Philosophical Transactions of Edinburgh, 11, 23. 
