ON MATTER AND FORCE. 25 



propagation of light, and of the way in which 

 different rays interfere with one another, and 

 the laws so deduced are abundantly confirmed 

 by experiment. But here also science kicks 

 down the ladder by which she has risen. In 

 order to explain the phenomena of light, it is 

 not necessary to assume anything more than a 

 periodical oscillation between two states at any 

 given point of space. What the two states are 

 nobody knows, and the only thing we can 

 assert with any degree of probability is that 

 they are not states of merely mechanical 

 displacement, like the tremor of a jelly, for the 

 phenomena of fluorescence appear to negative 

 this supposition.'* 



Of the existence of this aether, spectrum 

 analysis has given us no evidence, yet this 

 most sensitive test has proved that some of the 

 substances that are on our earth exist also in 

 the stars, nebulae, and comets. As yet no great 

 opportunity for analysing the matter of comets 

 has occurred. It is probable that nitrogen and 

 the aerolitic metals exist in them. When we 

 are told that the tail of the comet of 1811 

 extended 112 million miles through space, and 

 that the nature of the matter composing it may 



