ETHER AND PONDERABLE MATTER 



state of the substance that the molecules collectively 

 compose whether, that is to say, it shall appear to 

 our blunt perceptions as a gas, a liquid, or a solid. In 

 the gaseous state, as we have seen, the translational 

 motion of the molecules is relatively enormous, the 

 molecules being widely separated. It does not follow, 

 as we formerly supposed, that this is evidence of a re- 

 pulsive power acting between the molecules. The phys- 

 icists of to-day, headed by Lord Kelvin, decline to 

 recognize any such power. They hold that the mole- 

 cules of a gas fly in straight lines by virtue of their in- 

 ertia, quite independently of one another, except at 

 times of collision, from which they rebound by virtue of 

 their elasticity ; or on an approach to collision, in which 

 latter case, coming within the range of mutual attrac- 

 tion, two molecules may circle about each other, as a 

 comet circles about the sun, then rush apart again, as 

 the comet rushes from the sun. 



It is obvious that the length of the mean free path of 

 the molecules of a gas may be increased indefinitely by 

 decreasing the number of the molecules themselves in a 

 circumscribed space. It has been shown by Professors 

 Tait and Dewar that a vacuum may be produced arti- 

 ficially of such a degree of rarefaction that the mean 

 free path of the remaining molecules is measurable in 

 inches. The calculation is based on experiments made 

 with the radiometer of Professor Crookes, an instru- 

 ment which in itself is held to demonstrate the truth of 

 the kinetic theory of gases. Such an attenuated gas 

 as this is considered by Professor Crookes as const 

 ing a fourth state of matter, which he terms ultni- 

 gaseous. 



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