1885.] on how ThoiigJd presents itself in Nature. 183 



the spectroscope, but it comes to us in an exceedingly fragmentary 

 form, and it is difficult to get definite information out of it. Perhaps 

 the most precise information we as yet possess is in the case of a 

 periodic motion which occurs within the molecules of a vapour called 

 chlorochromic anhydride, which motion has been found to be repeated 

 about 809,520,000000 of times every second, and to be of such a kind 

 as to bear a close analogy to the motion of a point on a violin string 

 which is nearly but not quite two-fifths of the length of the string 

 from one end. 



To return to the air about us, in which the molecules are at about 

 a ninth-metret asunder: it is only when they happen to get much 

 nearer together than this ninth-metret that they can interfere with 

 each other's motions, and the mean length of their free paths, the 

 average distance to which they are able to make their way between 

 their encounters is somewhere about three-quarters of a seventh- 

 metret ; from which and from the speed with which they are moving 

 it is computed that each molecule usually is subjected to about 

 7000,000000 encounters in each second. 



Accordingly we may get some picture of the path pursued by a 

 molecule within one second of time by imagining a line 500 metres 

 long (the length of Grosvenor Place or Portland Place, or twice the 

 length of Albemarle Street) crumpled up till it has 7000 millions of 

 angles upon it ; the little straight bits will be of various lengths, all 

 small, and on the average about three-quarters of a seventh-metret 

 long, i. e. about one-third of the diameter of the smallest spec that 

 can be seen with the best microscope. 



But I do not know of a contrivance by which we can form any 

 adequate conception of the far more dainty little motions within the 

 molecules, upon which one-third of their energy is expended. 



Liquids and Solids. 



When the gas is condensed into a liquid or frozen into a solid, 

 the molecules are forced close together. In the liquid state they can 

 still jostle about among one another though there is not room for 

 free paths : in fact they are always in a state of encounter. And if 

 the liquid is frozen into a solid the molecules become so much 

 restrained that they cannot get past each other and are unable to 

 travel away from the place assigned to them. However, the delicate 

 and almost immeasurably rapid internal motions still go on, either 

 in the same form as before, or, as more usually happens, in a modified 

 form. It is these which start vibrations around, the waves of light 

 and radiant heat, with various periodic times, the swiftest that have 

 been measured by the spectroscope in the ultra-violet rays being 

 repeated about as often each second as there are seconds in 50 millions 

 of years, the slowest that are visible to our eyes being repeated as 

 often as there are seconds in 12 millions of years, and still slower 

 ones affecting us as heat though we have not such substances in the 

 pigment of our eyes as would be influenced by them as light. 



