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XXXI. Remarks on apaper "On Ice and Glaciers" in thelast Num- 

 ber of the Philosophical Magazine. In a letter to Prof. Tyndall. 

 By Prof. J. D. Forbes. 



My dear Sir, 



IN the part of your short paper " On Ice and Glaciers/' in the 

 Philosophical Magazine for February, in which my writings 

 are referred to, I think that my meaning has been misappre- 

 hended. Perhaps I can very briefly state what, in each of two 

 cases, I conceive to be a needless source of difficulty uninten- 

 tionally entertained by you with reference to my theoretical views 

 or explanations. 



The explanations which I now offer will tend, I hope, towards 

 a reconcilement of our conclusions, and not to controversy. 



I. With reference to the theory of " Regelation," you say, on 

 p. 93 of the Phil. Mag. for February, — "Why is the 'rapid 

 pounding ' necessary in the experiment of Prof. Forbes ? Doubt- 

 Jess 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 ther- 

 mometer will not show the smallest fraction of a degree below 

 32°, may, with the utmost facility, be converted by pressure into 

 solid ice." 



And again, " 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 con- 

 verted by pressure into solid slabs of ice.'' 



According to my view, in the former of these cases the ther- 

 mometer can only show the temperature of the ice when the 

 latter is freshly pounded. If kept at a thawing temperatm'e, each 

 icy fragment becomes invested with a film of water which has 

 the temperature of water, not of ice. You say, " Give the ice 

 time to rise to 32°." I answer, that it is not in the nature of 

 ICE to do so. The heat communicated becomes latent in the 

 water produced, not one iota of it reaching the ice, which, whilst 

 it remains Ice, is incapable of having its temperature raised. 

 Precisely the same consideration applies to the second paragraph. 

 Granulate the ice as we will, whilst any Ice remains, it has the 

 temperature of Ice, not of Water. The powder or fine slush which 

 you describe has the temjjerature of 32°, because the thei-mo- 

 raeter is in contact with the water alone, which forms so large a 

 part of its composition. A thermometer is, in short, incapable 



