1 825.] on the Process of Combustion . 347 



sphere of action, and thus offer considerable resistance to the 

 chemical union of the two elements. Further it may be sup- 

 posed, that when combustion has become very brisk, as it is 

 termed, the attraction between the combustible material and the 

 oxygen shall have become so energetic as to suspend or altoge- 

 ther to overcome the deoxidizing power of the sun's rays. Nay, 

 that those very rays which, at a less elevated temperature, had 

 but a few moments before retarded the process, may now contri- 

 bute materially to its acceleration. In this respect we merely 

 assign to them a property equivalent to that unquestionably 

 possessed by the calorific rays under particular circumstances. 

 In several instances these last mentioned rays have the power, 

 when of a certain degree of intensity, of causing the union of 

 several bases with oxygen, while at a more elevated range of 

 temperature, they will have the effect of occasioning their total 

 disseveration. Perhaps the red oxide of mercury affords us one 

 of the most remarkable and satisfactory instances of this circum- 

 stance. At the temperature of 600°, mercury will combine with 

 about 8 per cent, of oxygen, forming an acrid caustic substance 

 consisting of brilliant, sparkhng, deep red scales; but if the heat 

 be raised even a few degrees beyond this, so far from combin- 

 ing with a still further proportion of oxygen, the whole of this 

 principle is released from its combination, and the metal returns 

 to its original state of fluidity. 



As the solar rays are now ascertained to consist of three dis- 

 tinct species of radiant matter ; namely, those that impart heat, 

 those that impart light, and the chemically acting rays, it 

 appeared to me that the best mode of putting the conjecture I 

 have ventured to advance to the test of experiment would be to 

 try whether any difference could be detected in the loss sustained 

 by a lighted taper when exposed to the several portions of the 

 solar spectrum. Accordingly I constructed an apparatus similar 

 to that described by Dr. Herschell in his interesting researches 

 on " the Power of the Prismatic Colours to heat and illuminate 

 Objects" consisting of a frame AB moveable on two centres, 

 into which I inserted a piece of pasteboard C D, having an open- 

 ing in it m n of sufficient size to allow the whole extent of one 

 of the prismatic colours to pass through. 



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