FREEZING OF ICE. 



151 



the inside of tea-kettles, steamboat boilers, &c. ; 

 also to the abundance o( 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 

 H 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. f 



* De la Beche. 



t Philosophical Transactions of Edinburgh, 11, 23. 



