346 Prof. Tyndall on some Physical Properties of Ice. 



A second plate, crowded with bubbles, was held as near to the 

 fire as the hand could bear. On withdrawing it, and examining 

 it through a pocket lens, the appearance was perfectly beautiful. 

 In many cases the bubbles appeared to be surrounded by a series 

 of concentric rings, the outer ring surrounding all the others 

 like a crimped frill. 



39. I could not obtain these effects by placing the ice in 

 contact with a plate of metal obscurely heated*, nor by the 

 radiation from an obscure source. Indeed ice, as before remarked, 

 is impervious to radiant heat from such a sourcef. The rays 

 from a common fire also are wholly absorbed near the surface 

 upon which they strike ; and hence the described internal lique- 

 faction was confined to a thin layer close to this surface. 



40. But not only does liquefaction occur in connexion with 

 the bubbles, but the " flowers," already described as produced 

 by the solar beams, start by hundreds into existence when a 

 slab of transparent ice is placed before a glowing coal fire. They, 

 however, are also confined to a thin stratum of the substance 

 close to the surface of incidence. In the experiments made in 

 this way, the central stars of the flowers were often bounded by 

 sinuous lines of great beauty. 



41 . The foregoing considerations show that liquefaction takes 

 place at the surface of a mass of ice at a lower temperature than 

 that required to liquefy the interior of the solid. At the surface, 

 the temperature 32° produces a vibration, to produce which 

 within the ice would necessitate a temperature of 32° + «, the 

 increment x being the additional temperature necessary to over- 

 come the resistance to liquefaction, arising from the action of 

 the molecules upon each other. 



42. Now let us suppose two pieces of ice at 32°, with moist- 

 ened surfaces, to be brought into contact with each other, we 

 thereby virtually transfer the touching portions of these pieces from 

 the surface to the interior, where 32 + a? is the melting temperatui-e. 

 Liquefaction will therefore be arrested at those surfaces. Before 

 being brought together, the surfaces had the motion of liquidity ; 

 but the interior of the ice has not this motion ; and as equilibrium 

 will soon set in between the masses on each side of the liquid 

 film and the film itself, the film will be reduced to a state of 



of the Atlas to the Systhne Glaciaire. In fig. 13 we have also a close re- 

 semblance of the flower-shaped figures produced by radiant heat in lake ice. 



* To develope water- cavities within ice, a considerable time is necessary ; 

 more time, indeed, than was sufficient to melt the entire pieces of ice made 

 use of in these contact experiments. 



f Hence the soundness of the ice under the moraines : the sun's rays 

 are converted into obscme heat by the overlying debris ; this only affects 

 a layer of infinitesimal depth, and cannot produce the disintegration of the 

 deeper ice, as the direct sunbeams can. 



