140 Revieio of the Controversy Regarding the Motion of Glaciers. 



to a higher temperature than 32°.* As soon as that point is reached, 

 it melts and flows down as water. Consequently, there always inter- 

 venes between the warm air and the mass of a glacier this superficial 

 film or layer of inconsiderable thickness, which acts as a barrier to the 

 free passage of conducted heat into the ice, and introduces a serious, if 

 not a fatal, objection to Mr. CroU's hypothesis. For we have just at 

 the point wliere he supposes conduction to take ^^lace, the exact con- 

 ditions that render it impossible. Instead of two bodies of widely dif- 

 ferent temperature, we have two whose difference in that respect is 

 infinitesimally small. The one — the mass of the glacier — is never per- 

 ceptibly below 32°. The other — the superficial layer of water of melt- 

 ing ice — never rises above that point. The physical difference between 

 them — ice and water — is striking ; but this difference does not require 

 more than an infinitesimal therraometric variation. The heat of 

 liquidity, though immense in amount, produces no effect on the tem- 

 perature, and can not, consequently, give rise to any wave of conduc- 

 tion between the two. 



It will no doubt be said in reply, that, as a fiict, heat is conducted 

 into the interior of ice, and shows itself by melting the ice at a dis- 

 tance from the surfiice. The experiments of Prof. Tyndall have con- 

 clusively determined this, and in so far seem to lend countenance to the 

 theory. "A jjiece of ice containing liquid cavities was placed in a freez- 

 ing mixture, and the liquid bubbles frozen ; it was then wrapped in 

 flannel and placed in a dark room, to exclude the effects of radiation. 

 Nevertheless, in two hours the liquid cavities were completely restored 

 by heat, which had been conducted through the substance without 

 visible prejudice to its solidity." f 



Tliat ice, therefore, like all other substances, is capable of conduct- 

 ing heat, is indubitable; but this same experiment of Prof. Tyndall's 

 shows the impossibility of accounting for the motion of a glacier by 

 means of this conduction. Though experiments are still wanting to 

 determine the exact conductivity of ice, yet it is certainly very low. 

 The greatest distance to which the heat was conducted was a quarter 

 of an inch, namely, to the middle of a half-inch plate of ice, and this 

 occupied two hours. J 



'■■■ We lay out of coiisitleration all jiassage of heat outward, and also all transmission inward, 

 when tiie surface has a toiupeiature of less than 32'-% because in neither case can the etl'ects 

 concern the question at issue. 



t Heat as a Mode of Motion, p. 318. 



X In this exceedingly low rate of conductivity we can see the reason for the slightness of 

 variation in the temperaf ure of a glacier. Unable to rise i)erceptibly above 32'' from its phys- 

 ical constitution, the ice is also unable to conduct the heat of its cbkniiel away fast enough to 

 fall mucli below it, and, consequently, observalioiis have never sbuwn a lower temper, tore than 

 about 31. u'' Fah. It is impossible, therefore, to admit, as Mr. Croll's tlicory requires, that the 

 ice of a glacier can conduct into its niasx from a supe.rficiaUaycr of ncurlij its men temperature 

 sufficient heat to produce any sensible effect at depths exceeding a few inches or feet. 



