of Edinburgh, Session 1875-76. 
141 
Monday , 20 th March 1876. 
Sir WILLIAM THOMSON, President, in the Chair. 
The following Communications were read : — 
1. On the Connection between Cohesion, Elasticity, Dilatation, 
and Temperature. By Professor George Forbes. 
(. Abstract .) 
At various times there have arisen supporters of one or other of 
two extreme hypothesis concerning the nature of what we define 
as force. These are the hypothesis of u action at a distance” and 
of “ no action at a distance.” 
According to the latter hypothesis, the centre of gravity of no 
body, however large or however small, can be moved from a posi- 
tion of rest, nor can its motion be altered in direction or amount, 
except by direct collision with another portion of matter. 
Starting from this supposition as a basis of argument, and without 
assuming anything further as to the manner in which the different 
physical forces are caused by collisions, it is possible to arrive at 
some very general theorems; and from these theorems conclusions 
may be drawn as the nature and connection of some of the physical 
forces, which are necessarily true if the hypothesis of no action at 
a distance be true. 
The principal result of these theorems is the following: — Let a 
rod be chosen of any substance whose cohesion and elasticity do 
not vary enormously with the temperature. Let a be its expan- 
sion, in terms of its length, when the temperature is raised 1°. 
Let /3 be the compression of the same rod, in terms of its length, 
when a unit weight is supported at its summit. Let c be the 
number of these units which, when suspended by the rod, suffice 
to break it by sudden rupture. Let 0 be the absolute temperature 
at which all these experiments are made. Then the theory leads us 
to the conclusion that 
Only a few experiments have been made by which we can test 
