ICE AND GLACIERS. 147 



ADDITIONS. 



THE theory of the regelation of ice has led to scientific discussions 

 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 accordingly 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, pres- 

 sure 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 Thomson'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 concentrated 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 are first in con- 

 tact, there will be a greater difference between the temperature 

 of the ice and water, and the bridges will be more rapidly 

 formed, and be of greater extent. 



