THE ETHER AND PONDERABLE MATTER 



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 constituting a 

 fourth state of matter, which he terms ultra-gaseous. 



If, on the other hand, a gas is subjected to pressure, 

 its molecules are crowded closer together, and the length 

 of their mean free path is thus lessened. Ultimately, the 

 pressure being sufficient, the molecules are practically 

 in continuous contact. Meantime the enormously in- 

 creased number of collisions has set the molecules more 

 and more actively vibrating, and the temperature of the 

 gas has increased, as, indeed, necessarily results in ac- 

 cordance with the law of the conservation of energy. 

 No amount of pressure, therefore, can suffice by itself to 

 reduce the gas to a liquid state. It is believed that 

 even at the centre of the sun, where the pressure is al- 

 most inconceivably great, all matter is to be regarded as 

 really gaseous, though the molecules must be so packed 

 together that the consistency is probably more like that 

 of a solid. 



If, however, coincidently with the application of press- 

 ure, opportunity be given for the excess of heat to be 

 dissipated to a colder surrounding medium, the mole- 

 cules, giving off their excess of energy, become relative- 

 ly quiescent, and at a certain stage the gas becomes a 

 liquid. The exact point at which this transformation 

 occurs, however, differs enormously for different sub- 



347 



