Modes of Physical Force. 99 



fact, the rule of correlation holds good with elec- 

 tricity no less than with the two modes of physical force 

 which have been already under consideration heat and 

 motion. 



Light and heat have so many points of agreement as 

 to make it difficult to conceive of disagreement in their 

 relation to matter. " Gases which have transmitted 

 light are altered, e.g., chlorine rendered capable of com- 

 bining with hydrogen, liquids are altered, per-oxalate of 

 iron is chemically changed and gives off carbonic acid, 

 and the light which had produced this effect is less 

 able to produce them a second time. Solids are altered 

 as is shown in the extensive range of photographic 

 effects." Fluorescence and phosphorescence may also 

 be reckoned as light writing itself as light upon material 

 bodies, and remaining legible for some time. The 

 luminosity is evidently connected with some material 

 change wrought by light, and not passing off at once. 

 Sometimes, as in the case of the soap-bubble, iridescence 

 has something to do with varying degrees of thickness 

 in the transparent film or plate upon which the ray of 

 light impinges, a given degree of thickness being con- 

 nected with a given colour : and always the colouring 

 matter of plants may be held to point to a material 

 foundation laid by the light. Light too cannot act upon 

 certain liquids and gases without producing those very 

 unmistakeable and intelligible material changes which 

 are distinguished as chemical. Thus, hydrocyanic acid 

 throws down a solid carbonaceous deposit, and is partly 

 decomposed in some other ways, when left exposed to 

 white light : and thus again, a mixture of chlorine and 

 hydrogen, which remains unaltered in the dark, soon 

 becomes hydrochloric acid in the light. As is seen in 

 this and many other instances it is more than difficult to 



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