1888-89.] Sir W. Thomson on Constitution of Matter. 693 
Molecular Constitution of Matter. By Sir William 
Thomson, 
(Read July 1, 1889.) 
§ 1 . The scientific world is practically unanimous in believing that 
all tangible or palpable matter, molar matter as we may call it, con- 
sists of groups of mutually interacting atoms or molecules. This 
molecular constitution of matter is essentially a deviation from 
homogeneousness of substance, and apparent homogeneousness of 
molar matter can only he homogeneousness in the aggregate. “ A 
body is called homogeneous when any two equal and similar parts 
of it, with corresponding lines parallel and turned towards the same 
parts, are undistinguishable from one another by any difference in 
quality.”* I now add that unless the “ part” of the body referred to 
consists of an enormously great number of molecules, this statement 
is essentially the definition of crystalline structure. It is, indeed, 
very difficult to imagine equilibrium, static or kinetic, in an irregular 
random crowd of molecules. Such a crowd might he a liquid, — I 
can scarcely see how it could be a solid. It seems, therefore, that a 
homogeneous isotropic solid is hut an isotropically macled crystal ; 
that is to say, a solid composed of crystalline portions having their 
crystalline axes or lines of symmetry distributed with random 
equality in all directions. The proved highly perfect optical 
isotropy of the glass of object-glasses of great refracting telescopes, 
and of good glass prisms, seems to demonstrate that the ultimate 
molecular structure is fine-grained enough to let there he homo- 
geneous crystalline portions, which contain very large numbers of 
molecules while their extent throughout space is very small in com- 
parison with the wave-length of light. 
§ 2. An ideal skeleton or framework for a homogeneous assemblage 
of bodies, or of material systems of any kind, or of qualities or pro- 
perties of any kind, distributed periodically throughout space, is 
defined and explained in § 45 ( a ) to (j) below, substantially 
* Thomson and Tait’s Treatise on Natural Philosophy, new edition, vol. i. 
part ii. §§ 675-678 ; or Elements of Natural Philosophy , §§ 646-649. 
VOL. XVI. 23/11/89 2 Y 
