182 Mr. G. Johnstone Stoney | FeK 6, 



direction in which the waves are propagated, while in light it is 

 transverse to that direction. But the greatest difterence is one to 

 which I have not yet drawn attention, viz. : that the motions in sound 

 and light belong to different orders of motion. To understand this, it 

 will be necessary to examine first other motions that are going on in 

 nature. 



Gases. 



We have gained more knowledge of the motions that go on in gases 

 than in either solids or liquids. A gas consists of separate little 

 missiles called its molecules, darting about in every conceivable 

 direction. In gas so perfect as the air about us at the surface of the 

 earth there is room enough between these molecules for each in its 

 flight usually to pass several of its fellows before its motion is 

 interfered with by its encountering any of them. When the encounter 

 takes place the two molecules that come together grapple with one 

 another in a peculiar way, sometimes exchanging part of their 

 contents, and always shaking up the internal motions that go on 

 within them both. After they get free from each other they dart off 

 in new directions, to be again turned aside when they encounter other 

 molecules. In this way the course of each is an irregular zig-zag, 

 consisting of little straight pieces corresponding to the free paths of 

 the molecule between its encounters. The rapidity of its flight is 

 liable to change at each encounter : by some encounters it is increased, 

 by others diminished, by a very excej)tional encounter it may possibly 

 be left the same as before. The length of each little free path will 

 of course depend upon how long the little traveller chances to avoid 

 its neighbours. 



The details of the motions cannot be separately traced, but never- 

 theless a good deal of valuable information about them has been 

 obtained and many useful averages have been determined. For ex- 

 ample, in the air about us, at the pressure and temperature which 

 prevail in this room, there are about a unit-eighteen, or 1 with 

 eighteen O's after it (1,000000,000000,000000, a million times a 

 million millions) of these molecules in each cubic millimetre, i. e. in 

 about the volume of a pin's head. If at any instant they could be 

 kept from darting about, and if their distances asunder were then 

 measured, the average of all these various little distances would be 

 about a ninth-metret, about as much as a child grows in the third of a 

 second. They dash about at various rates, some more others less 

 than 500 metres per second, but on the average at about that speed. 

 This is about the quickest speed with which modern ordnance can 

 launch their projectiles. It is not far short of a third of a mile in a 

 second. 



Of the available energy that is in the gas somewhat less than 

 two-thirds takes the form of this great activity ; the rest, which as 

 more than one-third, is occupied in maintaining internal motions that 

 go on within the molecules of the gas. About these internal motions 

 we have much information given to us through observations made with 



