230 ON THE PROPERTIES OF ICE NEAR ITS MELTING POINT. [1858. 



I. This explains the permanent lower temperature of the 

 interior of ice. 



Let AB be the surface of a block of ice contained in water 



d 



Ice 



N 



M 



Water 



Water 



M' 



L 1 



Fig. 25. 



at what is called a freezing temperature. That temperature is 

 marked by the level of the line QP above some arbitrary zero. 

 LM is, in like manner, the permanent but somewhat lower tem- 

 perature possessed by the interior of the ice. The space, partly 

 water, partly ice, or partaking of the nature of each, MNOP, 

 has a temperature which varies from point to point, the portion 

 NO corresponding to what may be called the physical surface 

 of the ice between AB and ab, which is " plastic ice," or " viscid 

 water," having the most rapid variation of local temperature. 



II. Such a state of temperature, though it is in one 

 sense permanent, is so by compensation of effects. Bodies of 

 -different temperatures cannot continue so without interaction. 

 The water must give off heat to the ice, but it spends it in an 

 insignificant thaw at the surface, which therefore wastes even 

 though the water be what is called ice cold, or having the tempe- 

 rature of a body of water inclosed in a cavity of ice.* 



* I incline to think that water, in these circumstances, may, though surrounded 

 by ice, have a fixed temperature somewhat higher than what is called 32. But I 

 have not yet had an opportunity of verifying the conjecture. 



(My idea is, that the invasion of cold from the surrounding ice is spent in 

 producing a very gradual " regelation " in the water which touches the ice, leaving 

 the interior water in possession of its full dose of latent heat, and also of a tempera- 

 ture which may slightly exceed 32. By similar reasoning, a small body of ice, in- 

 closed in a large mass of water, will preserve its proper internal temperature beloio 

 32 ; but, instead of regelation taking place, the surface is being gradually thawed. 

 This is the case contemplated in the paragraph of the text to which this note refers.) 



N.B.- The words in brackets were added to this note during printing. 13th 

 May 1858. 



