On Underground Temperatures. 



45 



was sufficient to fuse the rocks and account for volcanic phenomena, 

 he nevertheless brought prominently forward the enormous heat- 

 producing power of the disturbances caused by this contraction. He 

 made a series of elaborate experiments to ascertain the force re- 

 quired to crush blocks of a given size (3 or 3J cubic metres), and 

 measured the work done by the estimated heat evolved by the crush- 

 ing of I cubic foot of several classes of rock by the number of cubic 

 feet of water at 32° F. converted into steam of one atmosphere, or 

 212° F. This method, although not perfectly satisfactory, is sufficient 

 to prove the essential fact that a mechanical disturbance of the rocks 

 may develop a large amount of heat. I must refer to his valuable 

 paper* for full details of his results. The following is an abstract 

 from his large table of experiments. 



Class of rock. 



Specific 

 gravity. 

 Water 

 = 1000. 



Weight 

 (pressure) 

 per square 

 inch at first 



yielding. 



Mean 

 pressure 

 at which 

 the cubes 

 were 

 com- 

 pletely 

 crushed. 



Tempera- 

 ture of 1 

 cubic foot 

 of rock 

 due to 

 work of 

 crushing. 



Number of 

 pounds of 



water at 

 32°, evapo- 

 rated into 



steam at 

 212°. 







lbs. 



lbs. 



Fahr.f 



lbs. 





2 -337 



1,620 



4,966 



8° 



0-288 



Ma gnesian Limestone .. 



2-571 



3,699 



16,333 



26 



0-9 



Coal-measure Sandstone . 



2-478 



10,970 



29,783 



86 



2-5 



Devonshire Marble 



2 -717 



11,708 



34,938 



114 



3-44 





2-859 



15,510 



41,590 



144 



4-51 





2-827 



24,039 



63,737 



213 



6-86 



Aberdeen grey Granite. . 



2-678 



16,868 



51.123 



155 



4-44 





2-594 



26,149 



69,786 



198 



5 22 



Thus with, the ordinary sedimentary rocks the crushing weight (or 

 that at which the blocks yield to pressure) is from 2^ to 15 J tons 

 per square inch of surface, while for the crystalline rocks it rises to 31 

 tons. The heat produced on the metal surroundings by the crushing 

 was in most cases easily perceptible to the hand, and was so great in 

 some of the granites and porphyries as to necessitate a delay for the 

 apparatus to cool. Both Mallet and Rankin were of opinion that " in 

 the crushing of a rigid material such as rock, almost the entire me- 

 chanical work (with the small residue of external work) reappears as 

 heat." If, therefore, the disturbance affecting the massive strata of a 

 great mountain range were sudden or of short duration, an intense 

 degree of heat might be rapidly developed ; but there is reason to 

 suppose that such movements have been of extreme slowness during 



* "Phil. Trans.," vol. 163 (1873), p. 147. 

 f Omitting fractions of a degree. 



