Prof. Tyndall on Ice and Glaciers. 93' 



the freezing together of masses of ice with moist surfaces. His 

 explanation of the effect is briefly this : — When one surface is 

 " brought up nearly to physical contact " with the others " there 

 is a double film of viscid water isolated between two ice surfaces 



colder than itself. Part of the sensible heat which it 



(the film) possesses is given to the neighbouring strata which 

 have less heat than itself, and the intercepted film of water in 

 the transition state becomes more or less perfect ice." 



He finds, in fact, by experiment, that " masses of strong ice 

 which had already for a long time been floating in unfrozen 

 water-casks, or kept for days in a thawing state, being rapidhj 

 pounded, showed a temperature of 0°'3 F. below the true freezing- 

 point." And "water being carefully frozen into a cylinder 

 several inches long, with the bulb of a thermometer in its axis, 

 and the cylinder being then gradually thawed, or allowed to lie 

 for a considerable time iu pounded ice at a thawing temperature, 

 showed also a temperature decidedly inferior to 32°, not less 

 than 0°-35 F." 



The cause here suggested, namely that the moist film is frozen 

 by the cold which the ice possesses, is that which first occurs 

 in seeking an explanation of the phrenomenon in question ; 

 and it was with reference to it that I instituted the long series 

 of experiments on artificial ice, recorded in my paper " On some 

 Physical Properties of Ice," and which lead to conclusions op- 

 posed to those of Prof. Forbes. 



Why is the "rapid pounding" necessary in the experiment 

 of Prof. Forbes ? Doubtless in order that the ice may be brought 

 into contact with the thermometer before its temperature has 

 risen to 32°. But give the ice time to rise to 32° ; let its last 

 residue of cold be abolished — the mass thus warmed, and in 

 which the finest thermometer will not show the smallest fraction 

 of a degree below 32°, may, with the utmost facility, be converted 

 by pressure into solid ice. 



Let the thawing surface of a mass of ice be scraped away, so 

 as to obtain a fine ice-powder possessing the temperature of that 

 surface. Let not the alleged magazine of cold within the ice be 

 at all called upon ; such a powder, or more properly fine slush, 

 the temperature of which no thermometer can show to be below 

 32°, may, as in the former case, be converted by pressure into 

 solid slabs of ice. 



Further, much of the ice which I made use of in my experi- 

 ments was full of cells partially filled with liquid water : the ice 

 containing them could not be lower than 32°, and was never 

 observed to be lower. Water, it is true, will remain liquid be- 

 low 32°; but normal ice, at the atinos])h(;ric pressure, cannot be 

 melted by a temperature below 32°; and I have shown that such 



