210 Mr. Morgan's Ohfefuat'.ons and Experiments on 



may be made as Inminous as ever by expofure to the fun. But 

 2^. It is obfervable, that fome bodies, which are moO: beau- 

 tifully phofphoric, or which, according to Mr. Wilson*s 

 theory, are in the bed Aate of flow combuftion ; it is obfervable, 

 I fay, that the fame bodies are the moftobftinate in refifting the 

 fire. The diamond, which to be decompofed requires the 

 force of a mofi: powerful furnace, is, according to this 

 theory, wailing away, owing to a feparation of parts which 

 is promoted by the weakeft influence of the fun's rays. — With- 

 out determining whether the p>recedmg obje6lions be valid, let us 

 now fee the confequence of admitting the common hypothecs, 

 that the detention of tbofe rays which fall upon phofphori is 

 owing to fome force which prevents their immediate reflection, 

 but is not adequate to their entire abforption. This force, 

 whatever it be, cannot well be fuppofed to operate with equal 

 power on all the rays. And if this be not the cafe, I think 

 we cannot avoid concluding, that phofphoric fhells will afliime 

 -r different colours, owing to the earlier and later efcape of the 

 different rays of light. This conclufion is juftified by an expe- 

 riment which I have already appealed to. When the force is 

 f ich as to admit of the efcape of the purple, the blue, and. the 

 green, we have only to leffen that force by warming the bodv, 

 and the yellow, the orange, and red efcape. It is proved by 

 Beccaria's extenfive experience on this fubj.ft, that there is 

 fjarcely any body which is not phofphoric, or which may. not 

 be made fo by heat. But as the phofphoric force is moft 

 powerful when the purple rays only efcape, fo we are to- con- 

 clude, that it is weakeft when it is able to retain the red rays only. 

 This conclufion is agreeable to fevcral faCls. Chalk, oyfter- 

 thells, together w^ith thofe phofphoric bodies whofe goodnefs 

 7 has 



