Mr. G. J. Stoncy on the Internal Motions of Gases. 133 



appear to consist of molecules moving about actively and irregu- 

 larly in all directions, the path of any one being for the most 

 part rectilinear, or, in other words, most of its motion being exe- 

 cuted at a sufficient distance from the neighbouring molecules 

 to be beyond the reach of sensible influence from them. Every 

 now and then, however, each molecule comes sufficiently near 

 some other molecule to have its course bent, on which occasions 

 it darts off in a new direction. Moreover many facts in physics 

 and chemistry lead irresistibly to the conclusion that the mole- 

 cules are resolvable into simpler elements; and the probability 

 distinctly is that each in most gases is a highly complex system. 

 When a body so constituted is enclosed, the molecules by fling- 

 ing themselves against the walls of the containing vessel produce 

 the pressure of the gas. If the enclosure be at the same tem- 

 perature as the gas, they do so without gain or loss of vis viva. 

 But if the wall be at a higher temperature, the activity of those 

 molecules which strike it is increased, and vice versa. The 

 altered activity is shared with the rest of the gas by conduction 

 and convection — or, more slowly, by conduction only, if the cir- 

 cumstances do not admit of convection ; and so the temperature 

 of the whole becomes changed. 



2. If the temperature of the gas be raised while the gas is 

 kept within a confined space, the vivacity of all the motions will 

 be thus increased ; but this will not affect d, the mean distance 

 at which the molecules are spaced, and scarcely affect I, the ave- 

 rage length to which a molecule can travel before it becomes 

 engaged with another molecule. If, on the other hand, the gas 



strates that no statical theory, whether on the hypothesis of a continuous 

 substance or of distinct particles, is possible. 



A gas is susceptible of enormous dilatation and compression without an 

 abrupt change in the laws upon which its pressure depends ; hence, if it 

 consist of particles at rest, the force which acts in any direction on any one 

 must be the result of forces emanating from many others, no one contribu- 

 ting more than a share, which may be regarded as infinitesimal. Hence it 

 is easy to see that if the density be changed, the pressure will vary as the 

 square of the density; for the force in any direction on any one particle 

 will increase as the number of the particles on that or the opposite side 

 (according as the elementary forces are attractive or repulsive) near enough 

 to act on it, i. e. will increase as the density ; and the number of particles 

 subjected to this augmented force which are found within each element of 

 volume will also have*increased in the same proportion. Hence the pres- 

 sure per square millimetre across any surface within the gas will increase 

 as the square of the density ; and as this is a law which does not exist in 

 gases, it follows that no gas consists of distinct particles at rest. The same 

 proof applies, by the principles of the Differential Calculus, to the hypo- 

 thesis of a continuous and homogeneous substance. For this proof, given 

 more at large, see Proceedings of the Royal Irish Academy, vol. vii. (1858; 

 p. 37. 



