HEAT. 719 



the polar circles, and only for a part of the surface within them. (Phil. Mag., V., vii., 

 381, 1879.) 



3. Chemical and Mechanical Action. — Heat is evolved by chemi- 

 cal changes in which there is condensation, as in liquids becoming sol- 

 ids, or gases, liquids, and in oxydation, etc. ; often an effect of the nat- 

 ural decomposition of minerals, or of vegetable or animal matter. The 

 oxydation of sulphides, and especially of the most common of them, 

 pyrite and pyrrhotite, is a source of heat in many mines, and for many 

 warm springs. In the formation of a pound of water from vapor, heat 

 enough is given out, says Tyndall, to melt five pounds of cast iron. 



Mechanical action, as the beating of waves on a coast, the falling 

 of water in cascades or rain, the shakings of earthquakes, sliding of 

 rocks, motion of the atmosphere in winds, produces heat, whenever 

 the action meets with resistance, on the principle that motion corre- 

 sponds to an amount of heat, or that heat is transformed motion. The 

 heat thus resulting is, however, of little geological importance. But 

 the friction attending uplifting, plicating, shoving along fractures, and 

 crushing of rocks, has often been an efficient and wide-reaching source 

 of heat and of geological work, producing among the earth's strata 

 consolidation, metamorphism, and probably even fusion. Mallet con- 

 cludes, from his calculations, that 7,200 cubic miles of crushed rock 

 would cause heat enough to make all the volcanic mountains of the 

 globe ; and that, since the ejections of volcanoes have been going for- 

 ward through a very long period, the action would require but an in- 

 finitesimal amount of annual crushing — not over 0*606 of a cubic 

 mile. 



Mallet demonstrates (Trans. Koy. Soc, 1872), by many careful experiments, that the 

 crushing of a cubic foot of syenyte or granite produces 119 to 213i degrees Fahrenheit; 

 of two slates, 13285 and 144-29 degrees; of three sandstones, 3284, 47 '79, 86'13 de- 

 grees; of two compact limestones, 20-98, 26*28 degrees; of Devonshire marble, 114*68 

 degrees. He obtained, for the specific heats of the same rocks, the syenytes and gran- 

 ites, 0181 to 196; slates, 201, 0*218; sandstones, 0233, 0*238, 0215 ; common lime- 

 ,-tone (being mostly in the state of aragonite, as shown by Sorby), 0-245, 0*265 ; marble 

 (or limestone in the state of calcite), 0-203. He concludes, that each cubic mile of rock 

 of the crust, taking it at the mean density, when crushed to powder develops sufficient 

 heat to melt 876 cubic miles of ice into water at 32° F., or to raise 7*600 cubic miles 

 of water from 42 D to 212° F., or, taking the average melting point of the rocks at 2,000° 

 F., to melt nearly 3£ cubic miles of such rock if of the same specific heat. Also that, 

 taking the amount of volcanic energy annually expended b\- our globe as less than one 

 fourth of the total telluric heat annually dissipated, this amount is represented by the 

 transformation into heat of the work of crushing about 247 cubic miles of (mean) rock, 

 which is not l-260,000,000th of the volume of the crust, supposing the crust a fourth of 

 the total volume of the entire globe. 



The facts prove that movements causing friction in the earth's rocks 

 have been an important source of heat and of geological change. But 

 it is not certain that there has been the crushing needed to make heat 



