Mr. Gr. J. Stoney on Polarization Stress in Gases. 407 



any width we please. Let us take a width equal to the diameter 

 of the smallest object that can be seen with a microscope, 

 which is about 2*5 seventh-metres, or the 100,000th part 

 of an inch. We have now to compare the dimensions of 

 this tube with the number and motions of the molecules in- 

 cluded within it. The number of molecules in a cubic milli- 

 metre of atmospheric air is about a unit-eighteen (10 18 ). (See 

 Phil. Mag. August 1868.) Whence the average interval be- 

 tween them is about a ninth-metre. This is the 100,000th 

 part of the length of our tiny tube and the 250th part of its 

 breadth. Hence the tube will contain a vast number of mole- 

 cules, some such number as five thousand millions. Again, 

 the average striking-distance (i. e. the average length of path 

 between the encounters) of the molecules is about the 1500th 

 part of the length of the tube, or the fourth, part of its breadth. 

 There is, therefore, abundant room within the tube, small as 

 it is, for a vast number of molecules and for much jostling 

 amongst them. The temperatures with which we are dealing 

 are such that the average velocity with which the molecules 

 of air dash about may be taken as 500 metres per second; 

 and the molecules meet with so many encounters, that the di- 

 rection of the path of each is changed somewhere about a 

 unit-ten of times (10,000,000,000) every second. To com- 

 plete the picture, we must remember that each molecule is in 

 a state of vigorous internal motion as well as travelling about 

 among its fellows, and that when an encounter takes place, 

 the energy which passes from one molecule to another is 

 employed in changing both those kinds of motion, and pos- 

 sibly (but not probably) another part becomes potential 

 energy, i. e. energy expended in altering the configuration of 

 the parts of the molecule, or the position of its parts with re- 

 ference to the aether. The motions which go on within the 

 molecules are what give rise to the linear spectra of gases, and 

 are therefore those motions of the gas that act on the aether, 

 and are in turn partly controlled by it*. They are recurring 

 motions which, at least in some cases, are resolvable either 



* May we not look, with some prospect of success, to the control which 

 is exercised by the sether on the internal motions of the molecule for the 

 explanation of the number of" degrees of freedom " of a molecule, which 

 (on the supposition that there is no potential energy) is in most gases 5 

 (see Watson's ' Kinetic Theory of Gases/ p. 3D J. The number 5 ap- 

 pears to indicate that the motions within the molecules are trammelled, 

 as here suggested. This view is, moreover, supported by the fact that 

 light is emitted by the gas, which could not be the case unless vast num- 

 bers of molecules moved in unison with one another -, and the most pro- 

 bable account of this appears to be that they are all trammelled in the 

 same way by their common relation to the asther. 



