396 Royal Society :— 
Thus the fact of regelation which Prof. Tyndall has taken as the 
basis of his theory for explaining the plasticity of ice, does in my 
opinion as much require explanation as does the plasticity of ice which 
it is applied to explain. The two observed phenomena, namely the 
tendency of the separate pieces of ice to unite when in contact, and 
the plasticity of ice, are indeed, as I believe, cognate results of a com- 
mon cause. ‘They do not explain one another. They both require 
explanation ; and that explanation, I consider, is the same for both, 
and is given by the theory I have myself offered. 
I now proceed to discuss the experiment by Prof. Forbes, already 
referred to as having been adduced in opposition to my theory. He 
states that mere contact without pressure is sufficient to produce the 
union of two pieces of moist ice *; and then states, as follows, his 
experiment by which he supposes that this is proved :—‘‘ Two slabs 
of ice, having their corresponding surfaces ground tolerably flat, were 
suspended in an inhabited room upon a horizontal glass rod passing 
through two holes in the plates of ice, so that the plane of the plates 
was vertical. Contact of the even surfaces was obtained by means of 
two very weak pieces of watch spring. In an hour and a half the 
cohesion was so complete, that, when violently broken in pieces, many 
portions of the plates (which had each a surface of twenty or more 
square inches) continued united. In fact it appeared as complete as 
in another experiment where similar surfaces were pressed together 
by weights.” He concludes that the effect of pressure in assisting 
‘regelation’ is principally or solely due to the larger surfaces of con- 
tact obtained by the moulding of the surfaces to one another. 
I have myself repeated this experiment, and have found the re- 
sults just described to be fully verified. It was not even necessary to 
apply the weak pieces of watch-spring, as I found that the pieces of 
ice, on being merely suspended on the glass rod in contact, would 
unite themselves strongly in a few hours. Now this fact I explain by 
the capillary forces of the film of interposed water as follows :—First, 
the film of water between the two slabs—being held up against gravity 
by the capillary tension, or contractile force, of its free upper surface, 
and being distended besides, against the atmospheric pressure, by the 
same contractile force of its free surface round its whole perimeter, 
except for a very small space at bottom, from which water trickles 
away, or is on the point of trickling away—exists under a pressure 
which, though increasing from above downwards, is everywhere, ex- 
cept at that little space at bottom, less than the atmospheric pres- 
sure. Hence the two slabs are urged towards one another by the ex- 
cess of the external atmospheric pressure above the internal water 
pressure, and are thus pressed against one another at their places of 
contact by a force quite notable in its amount. If, for instance, be- 
tween the two slabs there be a film of water of such size and form as 
might be represented by a film one inch square, with its upper and 
lower edges horizontal, and with water trickling from its lower edge, 
it is easy to show that the slabs will be pressed together by a force 
* “On some Properties of Ice near its Melting-Point,” by Prof. Forbes, Phil. 
Mag. 1858, vol. xvi. p. 544. 
