2 FOUNDATIONS OF THE HYPOTHESIS 7 



forces due to this motion are equilibrated by those of 

 cohesion. 



The difficulty of this problem disappears in the special 

 case which is the subject of this work. 



3. Behaviour of G-ases 



In gaseous bodies scarcely a trace of cohesion can be 

 found. In these most attenuated of all known media the 

 molecules seem so far apart that one experiences no attrac- 

 tion by another, except in the rarely occurring case of two 

 molecules coming accidentally very close, or even into colli- 

 sion, in consequence of their motion. 



This theoretical view explains in the simplest manner 

 the tendency of gases to expand, and it has a further sup- 

 port, derived from experiment, in the thermal behaviour of 

 gases when changing volume. For if a gas expands without 

 overcoming pressure, and therefore doing work if, for in- 

 stance, it streams into vacuous space its temperature falls 

 sp little that for long it was admitted, on the evidence of 

 tr ay -L us sac's experiments, that under such circumstances 

 no fall of temperature at all occurs. 



This behaviour would not be possible if on expansion a 

 gas had to overcome any considerable cohesion, since for 

 this an expenditure of energy, and therefore of heat, would 

 be requisite. Just as little can the assumption of repulsive 

 forces between the molecules be reconciled with this ex- 

 periment, since such forces would come into play during 

 expansion and generate energy in the shape of an increase 

 of heat in the gas. 



By the more exact experiments made by Joule and 

 Lord Kelvin on the heat-effects of gases in motion, it is, 

 indeed, shown that there is cohesion between the particles 

 even of gases; but the above conclusions are not thereby 

 invalidated, since Joule and Lord Kelvin's values for the 

 work done by an expanding gas in overcoming its own 

 cohesion are of nearly vanishing magnitude. 



It is specially important for our theory to note that all 

 experiments that have been made to determine this small 



