32 THE PRINCIPLES OF SCIENCE. 



of the magnet was cut off, and vice versd the light ap- 

 peared when the current was re-made. ' These pheno- 

 mena,' he says, * could be reversed at pleasure, and at any 

 instant of time, and upon any occasion, showing a perfect 

 dependence of cause and effect 11 / 



Another elegant experiment by Faraday illustrates the 

 maintainance of similar conditions. He proved that 

 liquids may conduct electricity when solids will not, by 

 putting the poles of a battery in melted nitre, when a 

 strong current was shown to exist by the galvanometer. 

 But as soon as the nitre was allowed to solidify, the 

 current ceased. Everything else remaining the same, the 

 current existed when the nitre was liquid, and not when 

 the nitre was solid *. 



It was Newton's omission to obtain the solar spectrum 

 under the simplest conditions which prevented him from 

 discovering the dark lines. Using a broad beam of light 

 which had passed through a round hole or a triangular 

 slit, he obtained a brilliant spectrum, but one in which 

 many different coloured rays overlapped each other. In 

 the recent history of the science of the spectrum, one 

 main difficulty has consisted in the mixture of the lines of 

 several different substances, which are usually to be found 

 in the light of any flame or spark. It is seldom possible 

 to obtain the light of any element in a perfectly simple 

 manner. Angstrom greatly advanced this branch of science 

 by examining the light of the electric spark when formed 

 between poles of various metals, and in the presence of 

 various gases. By varying the pole alone, or the gaseous 

 medium alone, he was able to discriminate correctly be- 

 tween the lines due to the metal and those due to the 

 surrounding gas k . 



h Experimental Researches in Electricity/ vol. iii. p. 4. 



i ' Life of Faraday,' vol. ii. p. 24. 



k < Philosophical Magazine/ 4th Series, vol. ix. p. 327. 



