52 Green Ice. 



mass. The colour rapidly diminished as the lumps were 

 reduced in size, and fragments an inch or two square only- 

 showed the tint where the little patches of converva actually 

 occurred. In some instances the vegetation was somewhat 

 more plentiful, and then the ice had to be reduced to still 

 smaller pieces before the green hue disappeared. 



I brought home several fragments of the ice in a bottle, 

 but as the weather was not so cold as on the former occasion, 

 about one third melted as I came along, and probably the con- 

 dition of the solid masses was changed. Small pieces were 

 placed on a strip of glass on the stage of the microscope, and 

 examined with a three-inch object-glass, the light being thrown 

 up strongly by means of the concave mirror. Under these 

 circumstances the ice appeared anything but homogeneous. 

 There were lots of bubbles, and a great confusion of optical 

 surfaces, bounding portions of different density, and portions 

 to which the crystalline structure gave a difference of refractive 

 power. The conferva seemed closely surrounded by unfrozen 

 water, and here and there a little air-bubble appeared, touching 

 the delicate green threads. Was the conferva left in a little 

 water-drop when the gelation took place, or, when thawing 

 began, did it take place first round the delicate plants ? 



In his Heat Considered as a Mode of Motion, p. 818, Pro- 

 fessor Tyndal gives a very interesting account of little water 

 chambers, with air-bubbles in them, which he found in Norway 

 ice, and he proved that they had been occasioned by melting 

 minute portions of the block. " If," said he, " the liquid is 

 the product of melted ice, its volume must be less than that 

 of the ice that produced it, and the associated air-bubble must 

 consist of rarified air." To test this, he melted some of the ice 

 in warm water, and found the air-bubbles shrink in volume at 

 the moment the surrounding ice was melted. In another expe- 

 riment he placed a portion of ice in a freezing mixture, and 

 froze the water-blebs. The ice thus treated "was immediately 

 placed in a dai-k 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 wjiter-blebs were perfectly restored in 



the centre of the slab of ice Hence no doubt can 



remain as to tlie possibility of effecting liquefaction in the 

 interior of a mass of ice by heat which lins passed by conduction 

 through the substance without melting it." Thus the existence 

 of water-blebs in ice does not prove that they consist of water 

 hl'l unfrozen when gelation took place; and I am disposed to 

 think that the conferva threads were, as they looked under a 

 hand-magnifier of low power, closely surrounded by frozen 

 water, but not frozen themselves, because their cell contents 



