CHAP. II.] PIONEERING DIFFICULTIES. 19' 



molecule. It is true that the molecule generally can travel but a very 

 small distance before its path is disturbed by an encounter with some 

 other molecule ; but after this encounter, there is nothing which 

 determines the molecule rather to return towards the place from 

 whence it came than to push its way into new regions. Hence in 

 liquids the path of a molecule is not confined within a limited region, as 

 in the case of solids, but may penetrate to any part of the space occu- 

 pied by the liquid. 



Now we have the motion of the molecule in the solid and the liquid.. 

 How about the movement in a gas 1 "A gaseous body is supposed to- 

 consist of a large number of molecules moving very rapidly." For in- 

 stance, the molecules of air travel about 20 miles in a minute. " During, 

 the greater part of their course these molecules are not acted upon by 

 any sensible force, and therefore move in straight lines with uniform 

 velocity. When two molecules come within a certain distance of each 

 other, a mutual action takes place between them which may be com- 

 pared to the collision of two billiard balls. Each molecule has its 

 course changed, and starts in a new path." 



The collision between two molecules is denned as an " encounter" ;. 

 the course of a molecule between encounters a " free path." " In 

 ordinary gases the free motion of a molecule takes up much more time 

 than is occupied by an encounter. As the density of the gas increases 

 the free path diminishes." 



It will be seen at once that on the view first held that the differ- 

 ence between continuous and discontinuous spectra depended simply 

 upon the solid and gaseous states, no solid could give us 'a line spec- 

 trum ; and the well-known absorption spectra of didymium glass and 

 other solid bodies would be impossible. 



Another important series of facts was soon brought to the front. 

 Pliicker and Hittorf in the year 1865, announced that "there is a 

 certain number of elementary substances which when differently 

 treated furnish two kinds of spectra of quite a different character, not 

 having any line or band in common." The difference in character to- 

 which reference is here made consists in the spectrum produced at the 

 lower temperature being composed of flutings, which are replaced by 

 lines when the higher temperature is reached. 



This was the first blow aimed at the general view one element 

 one spectrum to which I have referred above. It was met in two- 

 way s. 



Taking the line spectrum as representing the true vibration of the 

 chemical unit, I have already shown that the continuous spectrum was 

 explained as due to its physical environment, the solid or liquid state. 

 This, then, had not to be considered from the chemical point of view. 



C 2 



