Prof. J. Thomson on Theories and Experiments regarding Ice. 393 
cation of this theory, appears to adopt it to some extent, and to endea- 
your to make its principles cooperate with the views he had previously 
founded on Mr. Faraday’s fact of so called “ regelation”*. 
Professor James D. Forbes adopts Person’s view, that the dissolu- 
tion of ice is a gradual, not a sudden process, and so far resembles 
the tardy liquefaction of fatty bodies or of the metals, which in 
melting pass through intermediate stages of softness or viscosity. He 
thinks that ice must essentially be colder than water in contact with 
it; that between the ice and the water there is a film varying in 
local temperature from side to side, which may be called plastic ice, 
or viscid water ;, and that through this film heat must be constantly 
passing from the water to the ice, and the ice must be wasting away, 
though the water be what is called ice-cold. 
There is a manifest difficulty in conceiving the possibility of the 
state of things here described: and I cannot help thinking that 
Professor Forbes has been himself in some degree sensible of the 
difficulty ; for in a note of later date by a few months than the paper 
itself, he amends the expression of his idea by a statement to the 
effect that if a small quantity of water be enclosed in a cavity in ice, 
it will undergo a gradual “ regelation ;”” that is, that the ice will in 
this case be gradually increased instead of wasted. In reference to 
the first case, I would ask,—What becomes of the cold of the ice, 
supposing there to be no communication with external objects by 
which heat might be added to or taken from the water and ice 
jointly considered? Does it go into the water and produce viscidity 
beyond the limit of the assumed thin film of viscid water at the sur- 
face of the ice? Precisely a corresponding question may be put re- 
latively to the second case—that of the large quantity of ice enclosing 
a small quantity of water in which the reverse process is assumed to 
occur. Next, let an intermediate case be considered—that of a me- 
dium quantity of water in contact with a medium quantity of ice, and 
in which no heat, nor cold, practically speaking, is communicated to 
the water or the ice from surrounding objects. This, it is to be ob- 
served, is no mere theoretical case, but a perfectly feasible one. The 
result, evidently, if the previously described theories be correct, ought 
to be that the mixture of ice and water ought to pass into the state 
of uniform viscidity. Prof. Forbes’s own words distinctly deny the 
permanence of the water and ice in contact in their two separate 
states, for he says, “ 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.”? Now the 
conclusion arrived at, namely, that a quantity of viscid water could be 
* I suppose the term regelation has been given by Prof. Tyndall as denoting the 
second, or mending stage in his theory of “fracture and regelation.” Congela- 
tion would seem to me the more proper word to use after fracture, as regelation 
implies previous melting. If my theory of melting by pressure and freezing again 
on relief of pressure be admitted, then the term regelation will come to be quite 
suitable for a part of the process of the union of the two pieces of ice, though not 
for the whole, which then ought to be designated as the process of melting and 
regelation. E 
