210 #n f6*tE»r£" 



Euler imagined, that he could draw from this phenomenon 



an argument against Newton's theory of the emission of 



light, and in favour of his theory of vibrations : conceiving, 



that the effect of the rays of the Sun on these bodies was 



toprodueein them vrhrathns, which continued for some 



Light not a timc> Q n the contrary i deduce from it an argument mili* 



<ystem ot vi- * B 



braiions. tating directly against his* theory : for he supposes, that the 



different cofdurs of li^lit are different kinds of vibrations; so 

 that when these bodies are exposed to rays of certain eo- 

 lours, whieh can produce vibrations of their own kind only, 

 the light afterward emitted by these bodies in the dark 

 must be of the same colour. Wow from the experiments of 

 Mr. Wilson, which i myself have seen, the light emitted 

 by a calcined Oyster shell is always the same, whatever was 

 the colour of the ray, to which it had been exposed. These 

 oyster shells acquire different properties in this respect, ac- 

 cording to the degree of calcination, or some other unknown 

 circumstance. I have seen them emit red, green, and yel- 

 low light: but they never shine again, each with its own 

 colour, except from the immediate action of the solar rays; 

 and each single coloured ray causes them to emit the same 

 colour, but more faintly, the ray of their oitin colour having 

 no advantage over the rest. Hence I infer, that the rays of 

 the Sun produce a decomposition in these calcined sub- 

 stances, in consequence of which they give out the light, 

 stuiciv that entered into their composition. The decomposition is 



thus accelerated, and goes on for some time in the dark ; 

 but without this it would go on too slowly, for the light 

 that is evolved to be perceptible ; though it does go on, 

 for the calcined oyster shells do not long retain this pro- 

 perty. 

 Comets may Though a greater proximity of the Sun may thus bo the 

 'oiiwTn^from cause *' a more rapid decomposition of the luciferous va- 

 ihemseWes ai pours that arise from comets, this upproximation canuot 

 different tunes. pro j uce t fo e same e fl* ect either on different comets, or on the 

 same comet on its return to its perihelion, unless the pro- 

 duction of these vapours go on constantly with the same 

 degree of intensity ; which there"' fe no reason to suppose ; 

 for there may be a great difference in this respect between 

 different comets, and in the same comet at different times. 



