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pear to be combined with combuftible bodies, with different 

 degrees of attradive force. For in combuflion we find, that 

 different bodies are difpofed to pait with different rays with 

 greater facil-ty ; but when the combuflion is fufficiently rapid, they 

 part with all the different coloured rays together, and the flame 

 is therefore white ; and this is what is called a white heat. 

 Dr. Fordyce in the Phil. Tranf. for 1776, tells us, that when the 

 heated fubftances are colourlefs, they firfl emit a red light; then 

 a red mixed with yellow, and laftly, with a great degree of heat, 

 a pure white. All this is wonderfully conformable to the refradlion 

 of light by tranfparent fubftances, which refradV, and therefore at- 

 trad the red light lefs, and confequently in combuftion part with it 

 more eafily. On the other hand I know it is generally believed, 

 that the light in combuftion proceeds from the air, but this circum- 

 ftance of the different colour of the light in different cafes, feems to 

 overturn this opinion ; for if vital air were oxygen diffolved in 

 caloric and light, then the oxygen being abforbed by the burning 

 body, the light extricated would in all cafes be of the fame 

 nature ; the greater or lefs rapidity of the combuftion would only 

 produce an extrication of a greater or lefs quantity of light, but 

 could not produce any variation in its nature, it being neceffarily 

 the fame in all cafes, to wit, that in which vital air is diffolved. But 

 the truth or falftiood of this reafoning will not affedl the validity 

 of the poCition. that the refrangibility of the rays of light cannot 

 depend on the different magnitude, denfity or velocity of the 

 particles. 



But 



