ADDITIONS 



THE theory of the regelation of ice has led to scientific dis- 

 cussions between Faraday and Tyndall on the one hand, 

 and James and Sir W. Thomson on the other. In the text 

 I have adopted the theory of the latter, and must now ac- 

 cordingly defend it. 



Faraday's experiments show that a very slight pressure, 

 not more than that produced by the capillarity of the layer 

 of water between two pieces of ice, is sufficient to freeze 

 them together. James Thomson observed that in Faraday's 

 experiments pressure which could freeze them together was 

 not utterly wanting. I have satisfied myself by my own 

 experiments that only very slight pressure is necessary. It 

 must, however, be remembered that the smaller the pressure 

 the longer will be the time required to freeze the two pieces, 

 and that then the junction will be very narrow and very 

 fragile. Both these points are readily explicable on Thom- 

 son's theory. For under a feeble pressure the difference in 

 temperature between ice and water will be very small, and 

 the latent heat will only be slowly abstracted from the 

 layers of water in contact with the pressed parts of the 

 ice, so that a long time is necessary before they freeze. We 

 must further take into account that we cannot in general 

 consider that the two surfaces are quite in contact; under 

 a feeble pressure which does not appreciably alter their 

 shape, they will only touch in what are practically three 

 points. A feeble total pressure on the pieces of ice con- 

 centrated on such narrow surfaces will always produce a 

 tolerably great local pressure under the influence of which 

 some ice will melt, and the water thus formed will freeze. 

 But the bridge which joins them will never be otherwise 

 than narrow. 



Under stronger pressure, which may more completely 

 alter the shape of the pieces of ice, and fit them against 

 each other, and which will melt more of the surfaces that 



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