Mr. J. Gill on the Dynamical Theory of Heat. 371 



repulsion is accounted for by the decrease of molecular distances, 

 and by the addition of heat unde? % constant volume the molecular 

 distances are also in effect decreased; for by the addition of heat 

 or molecular vis viva, the particles of a gas moving with increased 

 energy would approach closer together at some points of their 

 orbits, and recede further asunder at the opposite points ; and 

 the general result of the increased energy of the molecular move- 

 ments would be an increase of tension if the repulsion increases, 

 in some geometrical ratio of the inverse distance, because the 

 repulsion would inmease by the closer approach of the particles at 

 their points of greater vicinity more than it would diminish by 

 the increased distance at the opposite points. Thus the tension of 

 elastic fluids (and the activity of thermo-dynamical phenomena 

 generally) may be referred to the common principle of mole- 

 cular repulsion, called into action by heat considered as mole- 

 cular motion. 



The motion of matter in masses may be stopped in various 

 ways, and transformed into different shapes of energy ; but it is 

 difficult to conceive how molecular motion could be stopped by 

 any arrangement or combination of common matter. Mole- 

 cular motion is being continually produced in natural phe- 

 nomena, and by artificial agencies, at the expense of energy in 

 various shapes. It can be transferred from' one body to another, 

 and modified indefinitely as to its intensity ; but its production 

 being continual, it must be supposed to accumulate indefi- 

 nitely, while the other forms of energy from which it has 

 its origin must at the same time be decreasing in equal pro- 

 portion — an assumption which is contrary to all rational proba- 

 bility. This difficulty does not exist in the dynamical theory of 

 heat, as this theory supposes a direct conversion of heat, or mole- 

 cular motion of common matter, into work in the same proportion 

 as work is actually proved to be convertible into heat or molecular 

 motion by friction ; and thus a continual and invariable circula- 

 tion of phenomena by interchange or transformation in terrestrial 

 dynamics can be satisfactorily accounted for, if the direct con- 

 version of heat into work can be proved to be true. 



Any theory of phenomena which involves the circumstance of 

 a continually accumulating amount of disturbance of the mobile 

 equilibrium requisite for the general permanence of physical ex- 

 istence must be considered unsatisfactory. The meteoric theory 

 of the sun's replenishment, independently imagined by Mayer 

 and by Mr. Waterston, and admirably developed by Professor W. 

 Thomson, is based on reasoning which might obtain for it uni- 

 versal favour, were it not for the idea it involves of a continual 

 transfer of matter in one direction only, and its consequent accu- 

 mulation in the sun, which is opposed to the ideas of constancy 



