7 
“ The question of discrete molecules is a perplexing one ; I 
“ mean rather the question whether fluid is to be supposed to 
“ consist of discrete molecules, or to be continuous. In a 
“ lecture (The Eeade Lecture), delivered last May by Professor 
“ Thomson, of Glasgow, before the University of Cambridge, he 
££ spoke earnestly against the idea prevalent among chemists, 
££ of the existence of indivisible molecules. This opinion would, 
<£ therefore, favor the possibility at least of fluid being really 
“ continuous.” 
“ If we assume the doctrine of discrete molecules, suppose 
££ them for instance to be spherical. Imagine them to be en- 
££ closed as in the diagram* in a £ rectangular’ vessel, the centres 
££ of the spheres lying in vertical lines. Now, the lateral pres- 
“ sure is arbitrary, depending upon the squeezing AB towards 
££ CD. The vertical pressure on BD is definite. This concep- 
£< tion would show the possibility, in unstable equilibrium, that 
££ pressure need not at a point of a fluid be equal in all directions. 
“ Of course the experiment could not be performed in unstable 
<£ equilibrium ; in stable equilibrium, where we can perform 
££ experiments, the fundamental principle of fluid pressure 
“ cannot be doubted.” 
This extract, and my paper, treat only of ordinary hydro- 
statics and have no reference to the kinetic theory of gases. 
How correct is the scientific attitude taken by Mr. Walton 
the controversy now or lately going on among chemists, res- 
pecting the legitimacy of the atomic hypothesis, may show. 
And, if I mistake not, this attitude is further justified by a 
question which has recently arisen respecting the constitution of 
the setherf of space. If Mr. Walton’s views have undergone 
any modification, I must take the whole blame for having- 
exhibited them in their unmodified form. One remark arises 
out of the above extract. Mr. Walton had elsewhere suggested 
that a slight jar to a spherical system in unstable equilibrium 
might, without disturbing the geometry of the fluid, transform 
the internal forces from a system of instability to one of stability, 
and place the internal pressure at each point in its orthodox 
state. In the case of ££ rectangular” unstable equilibrium, a 
slight jar might disturb the geometry of the fluid. J 
# The diagram is easily drawn. BD represents the horizontal base and 
BA and DC, drawn upwards, the vertical sides. Within the figure, left open 
at the top, are twenty equal circles lying in five horizontal rows of four. The 
diameter of each circle is one-fourth of BD. 
f Following Professor Joseph Bayma (see the footnote at p. 173 of his 
Elements of Molecular Mechanics, 1866,) I write tether to make a distinction 
between the luminiferous sether, or the aether of space, and the ethers of 
chemistry. Professor Challis had, at least as early as January, 1861, used 
the spelling cether in reference to the aether of space. 
£ If we take two or more sets of spheres, all those of one set being equal 
to one another and unequal to any sphere of another set, then the relative 
magnitudes may be so adjusted as that a slight jar to a system in * rectangular’ 
equilibrium shall give rise to arrangements which, under certain conditions, 
shall be definite and stable, 
