344 Prof. Tyndall on some Physical Properties of Ice. 



of air at the top to escape. At the moment the melting reached 

 the cavity the air-bubble instantly collapsed to a sphere, pos- 

 sessing, in some cases, far less than the hundredth part of its 

 original volume. The experiments were repeated with several 

 distinct masses of ice, and always with the same I'esult. I think, 

 therefore, it may be regarded as certain that the liquid cells are 

 the product of melted ice *. 



33. Considering the manner in which ice imported into this 

 country is protected from the solar rays, I think we must infer 

 that in the specimens examined by mef, the ice in contact with 

 the bubble has been melted by heat which has been conducted through 

 the substance without visible prejudice to its solidity. 



34. Paradoxical as this may appear, I think it is no more than 

 might reasonably be expected from a jmori considerations. The 

 heat of a body is referred, at the present day, to a motion of its 

 particles. When this motion reaches such an intensity as to 

 liberate sufficiently the particles of a solid from their mutual 

 attractions, the body passes into the liquid condition. Now as 

 regards the amount of motion necessary to produce this liberty 

 of liquidity, the particles at the surface of a mass of ice must be 

 very differently circumstanced from those in the interior, which 

 are influenced and controlled on every side by other particles. 

 But if we suppose a cavity to exist within the mass, the particles 

 bounding that cavity will be in a state resembling that of the 

 particles at the surface ; and by the removal of all opposing action 

 on one side, the molecules may be liberated by a force which the 

 surrounding mass has transmittedwithout prejudice to its solidity. 

 Supposing, for example, that solidity is limited by molecular 

 vibrations of a certain amplitude, those at the surface of the 

 internal cavity may exceed this, while those between the cavity 

 and the external surface of the ice may, by their reciprocal actions, 

 be preserved within it, just as the terminal member of a series 

 of elastic balls is detached by a force which has been transmitted 

 by the other members of the series without visible separation J. 



35. Where, however, experiment is within reach, we ought not 

 to trust to speculation ; and I was particularly anxious to obtain 

 an unequivocal reply to the question whether an interior portion 

 of a mass of ice could be melted by heat which had passed through 

 the substance by the process of conduction. A piece of Norway 

 ice, containing a great number of the liquid discs already 



* This of course refers to the lake ice examined as described. I venture 

 to predict, however, that the same will be found true of the bubbles in 

 glacier ice. 



t And in those portions of glacier ice which are withdrawn from the 

 direct action of the sun. 



X Of course I intend this to help the conception merely. 



