Prof. J. Thomson on Theories and Experiments regarding Ice. 395 
causes the change. Exemplifying, in another direction, the principle 
he is propounding, he refers to the fact that water may be exalted to 
the temperature of 270° Fahr., at the ordinary pressure of the atmo- 
sphere, and yet remain water; but that the introduction of the 
smallest particle of air or steam will cause it to explode, and at the 
same time to fall in temperature. He further alludes to numerous 
other substances—such as acetic acid, sulphur, phosphorus, alcohol, 
sulphuric acid, ether, and camphine—which manifest like phenomena 
at theirfreezing- or boiling-points, to those referred to as occurring with 
the substance of water, ice, and steam; and he adverts to the ob- 
served fact that the contact of extraneous substances with the parti- 
cles of a fluid usually sets these particles free to change their state, in 
consequence, he says, of the cohesion between them and the fluid 
being imperfect; and he instances that glass will permit water to 
boil in contact with it at 212° Fahr., or by preparation can be made 
so that water will remain in contact with it at 270° Fahr. without 
going off into steam; also that glass can be prepared so that water 
will remain in contact with it at 22° Fahr. without solidification, but 
that an ordinary piece of glass will set the water off at once to freeze. 
He afterwards comes to a point in his reasoning which he admits 
may be considered as an assumption. It is “that many particles in 
a given state exert a greater sum of their peculiar cohesive force 
upon a given particle of the like substance in another state than few 
can do; and that as a consequence a water particle with ice on one 
side, and water on the other, is not so apt to become solid as with 
ice on both sides ; also that a particle of ice at the surface of a mass 
[of ice] in water is not so apt to remain ice as when, being within the 
mass there is ice on all sides, temperature remaining the same.” 
This supposition evidently contains two very distinct hypotheses. 
The former, which has to do with ice and water present together, I 
certainly do regard as an assumption, unsupported by any of the 
phenomena which Mr, Faraday has adduced. The other, which has 
to do with a particle of ice in the middle of continuous ice, and 
which assumes that it will not so readily change to water, as another 
particle of ice in contact with water, I think is to be accepted as pro- 
bably true. I think the general bearing of all the phenomena he has 
adduced is to show that the particles of a substance when existing all 
in one state only, and in continuous contact with one another, or in 
contact only under special circumstances with other substances, ex- 
perience a difficulty of making a beginning of their change of state, 
whether from liquid to solid, ‘or from liquid to gaseous, or probably 
also from solid to liquid: but I do not think anything has been ad- 
duced showing a like difficulty as to their undergoing a change of 
state, when the substance is present in the two states already, or 
when a beginning of the change has already been made. I think 
that when water and ice are present together, their freedom to 
change their state on the slightest addition or abstraction of heat, or 
the slightest change of pressure, is perfect. I therefore cannot 
admit the yalidity of Mr. Faraday’s mode of accounting for the 
phenomena of regelation. 
