SECT. XXVII. PROPAGATION OF HEAT. 273 



in the direction of its axis of double refraction (N. 205), but 

 at right angles to that axis it is contracted, which brings 

 the crystal nearer to the form of the cube and diminishes its 

 double refractive power. When heat is applied to crystals of 

 sulphate of lime, the two optical axes (N. 207) gradually approach, 

 and at last coincide ; when the heat is increased, the axes open 

 again, but in a direction at right angles to their former position. 

 By experiment M. Scnarmont has concluded, that in media con- 

 stituted like crystals of the rhomboidal (N. 169) system the 

 conducting power varies in such a manner, that, supposing a 

 centre of heat to exist within them, and the medium to be inde- 

 finitely extended in all directions, the isothermal surfaces are 

 concentric ellipsoids of revolution round the axes of symmetry, 

 or at least surfaces differing but little from them. The internal 

 structure of crystallized matter must be very peculiar thus to 

 modify the expansive power of heat. 



Heat applied to the surface of a fluid is propagated downwards 

 very slowly, the warmer, and consequently lighter strata, always 

 remaining at the top. This is the reason why the water at the 

 bottom of lakes fed from Alpine chains is so cold ; for the heat 

 of the sun is transfused but a little way below the surface. 

 When the heat is applied below a liquid, the particles con- 

 tinually rise as they become specifically lighter, and diffuse the 

 heat through the mass, their place being perpetually supplied by 

 those that are more dense. The power of conducting heat varies 

 materially in different liquids. Mercury conducts twice as fast 

 as an equal bulk of water, and therefore it appears to be very 

 cold. A hot body diffuses its heat in the air by a double 

 process : the air in contact with it becoming lighter ascends and 

 scatters its heat by transmission, while at the same time 

 another portion is discharged in straight lines by the radiating 

 power of the surface. Hence a substance cools more rapidly 

 in air than in vacuo, because in the latter case the process is 

 carried on by radiation alone. It is probable that the earth 

 having been originally of very high temperature has become 

 cooler by radiation alone, the ethereal medium being too rare to 

 carry off much heat by contact. 



Heat is propagated with more or less rapidity through all 

 bodies ; air is the worst conductor, and consequently mitigates 

 the severity of cold climates by preserving the heat impartecj 



N 3 



