58 Scientific Proceedings, Royal Dublin Society. 



of a weight placed upon the piston, is to reduce the vapour to a 

 liquid, thereby bringing about a great diminution of volume and 

 proportional loss of gravitational potential by the weight. But 

 this change will by no means be brought about instantaneously. 

 When a little of the vapour is condensed, this portion parts with 

 latent heat of vaporisation, increasing the tension of the remainder, 

 or raising its point of saturation, so that before the weight descends 

 any further, this heat has to escape from the cylinder. 



Many more such cases might be cited. The heating of india- 

 rubber when expanded, its cooling when compressed, is a remark- 

 able one; for at first sight it appears as if this must render it 

 exceptional to the general law, most substances exhibiting the 

 opposite thermal effects when stressed. However, here, too, the 

 action of the stress is opposed by the secondary effects developed 

 in the substance ; for it is found that this substance contracts when 

 heated, expands when cooled. Again, ice being a substance which 

 contracts in melting, the effect of pressure is to facilitate melting, 

 lowering its freezing point. But so soon as a little melting occurs, 

 the resulting liquid calls on the residual ice for an amount of heat 

 equivalent to the latent heat of liquefaction, and so by cooling the 

 whole, retards the change. 



Clerk MaxwelP observes on the general principle that less force 

 is required to produce a change in a body when the change is un- 

 opposed by constraints than when it is subjected to such. From 

 this if we assume the external forces acting upon a system never 

 to rise above a certain potential (which is the order of nature, 

 irresistible forces are not the order of nature), the constraints of 

 secondary actions may, under certain circumstances, lead to final 

 rejection of some of the energy, or, in any case, to retardation of 

 change in the system — dissipation of energy being the result.^ 



As such constraints seem inherently present in the properties 

 of matter, it appears safe to summarise as follows : — 



The transfer of energy into any inanimate material system is 

 attended by effects retardative to the transfer and conducive to dis- 

 sipation. 



1 "Theory of Heat," p. 131. 



"^ The law of Least Action, which has been applied, not alone in optics, but in many 

 . mechanical systems, appears physically based upon the restraint and retardation opposing 

 the transfer of energy in material systems. 



