155 



test of hardness ; as it depends in a great mea- 

 sure on the form of the fragments, and, often 

 also, occurs in rocks otherwise of very inferior 

 hardness ; arising, in this case, from the inter- 

 mixture of quartz. 



FRANGIBIL1TY. 



The degree in which different rocks resist 

 the effect of a blow, is infinitely various, and 

 is independent of their hardness. Thus, lydiari 

 stone cannot endure the slightest blow with- 

 out breaking ; whereas potstone is difficultly 

 broken by any force. Fortunately no technical 

 phraseology has been invented to express these 

 variations, and they may be rendered intelligible 

 by comparison ^with known objects, and by the 

 use of common language. 



It must be recollected, in making such trials, 

 that the power of a hammer in splitting a rock, 

 or in detaching a fragment, depends more on its 

 impulse than its weight. 



It must also be recollected, that the frangi- 

 bility of rocks varies in proportion to the water 



