Prof. Tyndall on some Physical Properties of Ice. 345 



described, and several cells of air and water, was enveloped in 

 tinfoil and placed in a mixture of pounded ice and salt. A few 

 minutes sufficed to freeze the discs to thin dusky circles, which 

 appeared, in some cases, to be foi'med of concentric rings, and 

 reminded me of the sections of certain agates. Looked at side- 

 ways, these discs were no thicker than a fine line. The water- 

 cells were also frozen, and the associated air-bubbles were greatly 

 diminished in size. I placed the mass of ice between me and 

 a gas-light, and observed it through a lens : after some time the 

 discs and water-cells showed signs of breaking up again. The 

 rings of the discs disappeared ; the contents seemed to aggregate 

 so as to form larger liquid spots, and finally, some of them were 

 reduced to clear transparent discs as before. 



36. But an objection to this experiment is, that the ice may 

 have been liquefied by the radiation from the lamp, and I have 

 experiments to describe which will show the justice of this objec- 

 tion. A rectangular slab, 1 inch thick, 3 inches long and 2 wide, 

 was therefore taken from a mass of Norway ice, in which the 

 associated air- and water-cells were very distinct. I enveloped 

 it in tinfoil and placed it in a freezing mixture. In about ten 

 minutes the water-blebs were completely frozen within the mass. 

 It was immediately placed in a dark room, where no radiant heat 

 could possibly affect it, and examined every quarter of an hour. 

 The dim frozen spots gradually broke up into little water parcels, 

 and in two hours the water-blebs were perfectly restored in the 

 centre of the slab of ice. When last examined, this plate was half 

 an inch thick, and the drops of liquid were seen right at its centre. 



37. A second piece, similarly frozen and wrapped up in flannel, 

 showed the same deportment. In an hour and a half the frozen 

 water surrounding the air-bubbles was restored to its liquid con- 

 dition. Hence no doubt can remain as to the possibility of 

 eflFecting liquefaction in the interior of a mass of ice, by heat which 

 has passed by conduction through the substancewithout melting it. 



38. I have already referred to the formation of the liquid 

 ca\'ities observed by M. Agassiz, when glacier ice was exposed 

 to the sun. The same effect may be produced by exposure to a 

 glowing coal fire. On the 21st and 22nd of November I thus 

 exposed plates of clear Wenham Lake ice, which contained some 

 scattered air-bubbles. At first the bubbles were sharply rounded, 

 and without any trace of water. Soon, however, those near the 

 surface, on which the radiant heat fell, appeared encircled by a 

 liquid ring, which expanded and finally became crimped r^-v. 



at its border, as shown in the adjacent figure. The vOj 

 crimping became more pronounced as the action was ^^^-^ 

 |icrmitted to continue*. 

 * The blebs observed in glauier ice also exhibit this loi'iu : see tig. 8, ])latc 6. 



