Review of the Contvoveny Regarding the Mut'ion of Glaciers. 143 



of ice, its diathermancy is exceedingly low. Melloni has shown, that 

 of the light and heat radiated from a lamp, and falling on the surface 

 of a plate of ice one tenth of an inch in thickness, 94 per cent, is ab- 

 sorbed, and only 6 per cent, emerges from the opposite fall, while of 

 the total radiation of incandescent platumm only one half per cent, is 

 transmitted through such a plate. Ice is, in tact, one of the most im- 

 pervious substances known to the obscure or peculiarly calorific rays, 

 and while their can be no doubt that the shorter and more luminous 

 vibrations can and do pass into the mass of the glacier, perhaps to its 

 very base, it is equally certain that the longer and less luminous ones 

 fail to penetrate it to any appreciable distance. They are almost 

 wholly absorbed in warming the superficial layer, if beloAV 32°, and in 

 melting it if not below.* Hence, the surface of a glacier when exposed 

 to the sun's rays is soft, the ice is called " rotten." Stream of ice cold 

 water runs over it in every direction, and the level of the glacier sinks 

 by "oblation," sometimes amounting to eight or nine feet in the year. 



The only effect of the solar radiation, therefore, is to aid in producing 

 the intermediate layer, of which mention was made above— a layer 

 whose temperature seldom exceeds 32°, Avhich is constantly being 

 renewed by liquefaction, as fast as it is removed, and from which, on 

 Mr. Croll's theory, incessant waves of liquidity are passing into a mas.s 

 of ice of the same temperature as ifsef. 



Another very large deduction must be made from the heat supposed 

 by Mr. Croll to be thus freely conducted into a glacier. The structure 

 of the ice is such, that it is full of blebs and cracks, all affording 

 internal surfaces, on reaching which the heat (admitting its free trans- 

 mission) will spend itself in melting the ice upon these surfaces and 

 enlarging the cavities, so as to- aid the heat of radiation in reducing 

 the superficial layer to a crumbling mass of several inches in thickness, 

 and feeding the rivulets that trickle over and through the glacier in 

 summer time. It is hardly possible that if any appreciable amount of 

 heat should succeed in entering the mass, it could escape meeting on 

 its passage with one of these cavities, where it would necessarily cease 

 to become heat of temperature, and become latent as heat of Uquiditi/. 

 Moreover, Mr. Croll's illustration of his own theory is open to a charge 

 of partiality. He says : 



" Let us observe what takes place, say, at the lower end of a glacier."' 

 He then supposes the tranfer of heat upward, melting each molecule 



* The intense blue color of the ice,when seen from below, is due to the absorption of thelouger 

 rays by the superficial layers, while the shorter (blue) ones are able to penetrate almost un- 

 checked. This effect may also be Increased to some extent by fluorescence. 



X. E. and Z>, Phil. Mag., March, 18G0. 



