1888 - 89 .] Sir W. Thomson on Constitution of Matter. 707 
clear that our original group which was a crystal and is now fluid 
will remain more dense than the surrounding atmosphere of free 
atoms until we have imparted to the group far more of energy than 
was required to dislodge its constituent atoms from configurations of 
equilibrium. There then is a mass of liquid surrounded by an 
atmosphere of its vapour, and in thermal equilibrium with the 
vapour if we cease the action on its atoms by which we imparted 
energy to it. A little farther consideration would no doubt give us 
the virtual surface-tension of the liquid exactly according to Laplace’s 
theory of capillary attraction ; hut we must not pause over this at 
present. 
§ 44. Recommence applying forces to the atoms of the group, now 
liquid, and introducing fresh atoms into the surrounding atmosphere. 
The density of the atmosphere becomes greater, while that of the 
group becomes less. Go on till the two densities become equal: 
thus we reach the Caignard de la Tour and Andrews’ critical point. 
If we continue now imparting energy to our original group, or to 
any of the atoms of the assemblage, we simply have a homogeneous 
assemblage in a state of homogeneous intestine commotion all through ; 
the Boscovich realisation of a fluid raised higher and higher above 
its critical temperature. 
On Molecular Tactics of Crystals and of the Artificial 
Twinning of Iceland Spar (§§ 45-60). 
§ 45. (a) . . . (j). Summary of Bravais ’ Doctrine of a Homo- 
geneous Assemblage of Bodies. 
(a) The bodies must he equal, similar, and homochiral.* 
( b ) They must he all similarly oriented. 
(c) They must he so distanced mutually that any point in one 
* This will be more easily and not less thoroughly understood from illustra- 
tions than from a definition in general terms. Of an externally symmetrical 
man, the two hands are allochirally similar. Either is the 'pervert of the 
other ; or they are mutual perverts. Two men of exactly equal and similar 
external figures would be allochirally similar if one holds out his right hand 
and the other his left ; homochirally similar if each holds out his right hand 
or each his left. (We ignore at present the monochiral anti-symmetry of 
one heart on one side ; of interior structure of intestinal canal not in the plane 
bisecting the exterior symmetric figure, &c., &c.). Looking to § (•£) below, we 
