ICE AND GLACIEKS. 131 



ADDITIONS. 



THE theory of the revelation 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 Thom- 

 son observed that in Faraday's experiments pressure which could 

 freeze them together was not utterly wanting. I have satisfied my- 

 self by my own experiments that only veiy slight pressure is necessary. 

 It must, however, be remembered that the smaller the pressure the 

 longerwill 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 contact, 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. 



