130 ESSAYS BIOGRAPHICAL AND CHEMICAL 



hydrogen would render air explosive (for water is formed 

 with explosive violence when hydrogen and oxygen com- 

 bine), yet traces of hydrogen may coexist with oxygen 

 without combination, except when the mixture is actually 

 in contact with a flame. 



These, however, are not the only constituents of the 

 atmosphere ; and in the following pages an account will 

 be given of certain phenomena which render it exceed- 

 ingly probable that still more ' subtile matter ' is on the 

 eve of discovery. 



In order to follow the course of events, it is first neces- 

 sary to devote some attention to the supposed nature of 

 light. Owing to its being perceived by our special organ 

 of sense, the eye, it early attracted attention. At first 

 believed to consist of corpuscles, shot out from the lumin- 

 ous body, it is now recognised as arising from the vibra- 

 tions of a medium pervading all space, termed ether ; and 

 the propagation of light takes place much as waves spread 

 in a pond, except in this : the particles of ether, unlike 

 the waves of water, are not restricted in their motion to 

 one plane, but the oscillations may take place in all direc- 

 tions at right angles to the direction of propagation. 

 There appears, however, to be no limit to the mode or 

 magnitude of the ethereal waves ; and though it cannot be 

 positively stated that the wave-motion ever takes place, 

 like sound waves, in the direction of propagation, still that 

 mode of propagation of waves is not excluded. It is cer- 

 tain, however, that such a mode of transmission does not 

 correspond with the nature of light, which consists wholly 

 of transverse vibrations. 



Just as it is possible to measure the distance between 

 the crests of the waves of the sea, so it is possible to deter- 

 mine the distance between the crests of the waves of light, 

 or in other words, to measure the wave-length of light. 

 And it has been discovered that all the visible rays of 



