1888 - 89 .] Sir W. Thomson on Constitution of Matter. 705 
kept constant, while the group is gaining by the accession, to it of 
more and more single atoms from without. But the exhaustion of 
potential energy due to the greater number falling into, than being 
thrown out from, the group would cause an augmentation of 
kinetic energy in the surrounding atmosphere of free atoms. To 
obviate this, let the atmosphere around the group be contained in a 
finite closed vessel, which, when left to itself, repels each atom that 
comes near enough to it, and sends it hack inwards with unchanged 
energy. Now let portions of this hounding surface he movable, 
and let them he so moved hy proper external appliances, that work 
shall be done upon them hy the impinging atoms to just such a 
degree as to keep the average kinetic energy of the free atoms 
constant. We have thus a Boscovichian realisation of a crystal of 
ice (hoar-frost) or other substance growing by condensation of a 
surrounding atmosphere of the same substance. The process in 
nature requires the abstraction of what is called the latent heat of 
the vapour to allow it to condense. This in our Boscovichian 
system is performed hy the arrangement for letting work he done 
outwards hy the moving parts of the boundary. 
§ 40. Even if there were no surrounding atmosphere of single 
moving atoms, our group, unless quite free from intestine com- 
motion, would occasionally throw off an atom in virtue of the 
chance concurrence of different sets of component vibrations at 
some of the outlying atoms. Now let there he just enough of 
atoms moving about in the space around the group to cause as 
many fallings-in as throwings-out of atoms, and with just enough of 
kinetic energy to neither gain nor lose energy in the surrounding 
atmosphere through these changes. This will also cause the average 
kinetic energy of the group to remain constant. Thus we have a 
crystal surrounded hy an atmosphere of vapour at its own tem- 
perature, and at the proper temperature to cause neither condensa- 
tion of the vapour nor evaporation of the solid. 
§ 41. Now hy somehow applying force to the atoms of the group 
increase their vibrational energy. We must, hy introducing atoms 
from the boundary, increase the density of the atmosphere around 
it to cause as many atoms to enter the group as are thrown off from 
it. Continue this process until the inter-atomic oscillations in the 
group become so great that the atoms begin to pass from one con- 
