VOL. LXXV.] PHILOSOPHICAL TRANSACTIONS. QjQ 



becoming phosphoric again when it has been sufficiently cooled. Besides, when 

 a phosphoric shell has been made very hot, and while it has continued so, I have 

 conveyed the most brilliant discharge of a battery over it without effect. In 

 other words, heat, or the very cause which promotes combustion in all other 

 instances, in this particular case puts an end to it. Mr. Wilson, in his Treatise 

 on Phosphori, has described an experiment similar to the preceding. But the 

 result he mentions is different from that mentioned here. However, from a 

 regard to his authority, I have so frequently repeated my trials that I cannot 

 justly suspect myself of any inaccuracy. 2°. When bodies are wasted by com- 

 bustion, they can never be made to re-assume the appearances which they pre- 

 viously displayed. No power can give to ashes the phenomena of a burnino- 

 coal. But phosphoric bodies are very different in this respect ; for a shell mav 

 be made to lose all its light by exposure to heat, and again may be made as 

 luminous as ever by exposure to the sun. But 3°. It is observable, that some 

 bodies, which are most beautifully phosphoric, or which, according to Mr. Wil- 

 son's theory, are in the best state of slow combustion ; it is observable, I say, 

 that the same bodies are the most obstinate in resisting the fire. The diamond, 

 which to be decomposed requires the force of a most powerful furnace, is, 

 according to this theory, wasting away, owing to a separation of parts which is 

 promoted by the weakest influence of the sun's rays. — Without determining 

 whether the preteding objections be valid, let us now see the consequence of 

 admitting the common hypothesis, that the detention of those rays which fall 

 on phosphori, is owing to some force which prevents their immediate reflection, 

 but is not adequate to their entire absorption. This force, whatever it be, can- 

 not well be supposed to operate with equal power on all the rays. And if this 

 be not the case, I think we cannot avoid concluding, that phosphoric shells will 

 assume different colours, owing to the earlier and later escape of the different 

 rays of light. This conclusion is justified by an experiment which I have 

 already appealed to. When the force is such as to admit of the escape of the 

 purple, the blue, and the green, we have only to lessen that force by warming 

 the body, and the yellow, the orange, and red escape. It is proved by Beccaria's 

 extensive experience on this subject, that there is scarcely any body which is not 

 phosphoric, or which may not be made so by heat. But as the phosphoric force 

 is most powerful when the purple rays only escape, so we are to conclude that it 

 is weakest when it is able to retain the red rays only. This conclusion is agree- 

 able to several facts. Chalk, oyster-shells, with those phosphoric bodies whose 

 goodness has been very much impaired by long keeping ; when finely powdered 

 and placed within the circuit of an electrical battery, will exhibit by their scat- 

 tered particles a shower of light; but these particles will appear reddish, or their 

 phosphoric power will be sufficient only to detain the yellow, orange, and red 



