210 Mr. Morgan's Ohfervatlons and Expertmeni^ on 



may be made as luminous as ever by expofure to the fun. Bat 

 3°. ft is obfervable, that fome bodies, which are moft beau-' 

 tl fully phofphorlc, or which, according to Mr. Wilson*s 

 theorv, are in the beft ftate of (low combuftion ; it is obfcrvable, 

 I fav, that the I'rme bodies are the moil obftinate in refifting the 

 /ire. The diamond, which to be decompoled requires the 

 force of a moll powerful furnace, is, according to this 

 theory, wafting away, owing to a reparation of parts which 

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

 out determining whether the preceding objedions be valid, let us 

 now fee the confequence of admitting the common hypothecs, 

 that the detention of thofe rays which fall upon phofphori is 

 owing to fome force which prevents their immediate refle(5lion, 

 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 (hells will aflumc- 

 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 

 fach as to admit of the efcape of the purple, the blue, and the 

 green, we have only to leflbn that force by warming the body, ■ 

 and the yellow, the orange, and red efcape. It is proved by-1 

 Bfccaria's extenfive experience on this fubjc61:, that there is'j 

 fcarcely 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- jj 

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

 This conclufion is agreeable to fevtral fads. Chalk, oyfter- 

 ihells, together with thofe phofphoric bodies whofe goodnefs 

 7 has 



