574 Light doss not depend on Vibration. [Book III. 



ing medium. 6thly, The texture of many bodies is 

 actually changed by expofure to the light. The juice 

 of a certain fhell-fifh contracts, it is well known, a 

 very fine purple colour, when permitted to imbibe the 

 rays of the fun ; and the ftronger the light is the more 

 perfect the colour. Pieces of cloth wetted with this 

 fluid become purple, even though inclofed in glafs, if 

 the folar light only is admitted } but the effect is to- 

 tally excluded by the intervention of the thinneft plates 

 of metal, which exclude the light. Some of the pre- 

 parations of filver alfo, fuch as luna cornea, will re- 

 main perfectly white, if covered from the light, but 

 contract a dark purple colour when expofed to it j and 

 'even the colour of plants is derived from the light, 

 fmce a plant which vegetates in darknefs will be per- 

 fectly white. As colour is imparted by light, fo it is 

 alfo deftroyed by it. It muft have fallen within the 

 obfervation of every reader, that filks, and other fluffs 

 of delicate colours, are greatly by the action of 



light. Experiments "have been made upon the fame 

 fluffs by expofmg them to both heat and moifture in 

 the dark, and alfo by expofmg them to the light in the 

 vacuum of an air pump, and it was found by all thefe 

 experiments, that the change of colour was to be af- 

 cribed to the action of light *. 7thly, With refpect 

 to the emiffion of light by diamonds and other ftones, 

 it is eafily accounted for upon other principles j and 

 the arguments founded upon the electric fpark not 

 being fenfibly diminifhed will meet with a fatisfactory* 



* <f It was conje&ured by fome, that the rays ef the fun di/J 

 perfed thofe parts of the bodies on which colours depended ; but 

 Bonzius obfcrved, that when ribbons were expofed on white -pa- 

 per, the colours vaniihed from buth their fides, but that nothing 

 could be found ncarth,e places where they had lain." Prie/ikft 

 Optics, 381. 



folntion 



