1825.] onthe Process of Combustion* ' 345 



practice to place screens of different kinds before the fire-place, 

 or to close the shutters of the apartment in order to prevent as 

 much as possible the access of light to the burning materials. 

 I was for a long time impressed with the belief that this was 

 merely a piece of popular prejudice, for which there existed no 

 rational foundation whatever, or that at furthest, the appearances 

 might be owing to the retina having become less sensible to the 

 comparatively feeble rays emitted by a body in a low state of 

 combustion while already under the influence of a stronger light.* 

 But as opinions so generally entertained usually rest more or less 

 on observation and experience, the best sources of evidence in 

 all such cases ; and as I was unable to procure any information 

 whatever on the subject from the several works on chemistry 

 which I have had an opportunity of consulting, I was induced, 

 during the late summer, when we had such an unusual succes- 

 sion of steady sunshine, to make the following experiments. 



Exper. 1 . — Two portions of green wax taper, each weighing 

 ten grains, were both ignited at the same moment ; one of them 

 I placed in a darkened room, the other I exposed to broad sun- 

 shine in the open air : thermometer in sun 78° Fahr. ; in room 

 67°: loss as follov/s : t , ti 



In five minutes that placed in sunshine lost. . . v'. . . 8^ grs. 



darkened room lost . . 9^ 



Exper, 2. — Two portions of taper, each weighing 23 grains, 

 were placed under similar circumstances, as in the former expe- 

 riment. 



In seven minutes that placed in sunshine lost ..... 10 grs. 



darkened room lost 11 



We here see, notwithstanding the higher temperature to which 

 the taper in sunshine was exposed, which must of course have 

 favoured the liquefaction of the wax, and consequently its ascent 

 in the wick, that during the short period of seven minutes, there 

 was a difference of loss amounting to not less than one grain. 



Exper. 3. — A common mould candle, fourteen inches in 

 length and three in circumference, was accurately divided into 

 inches, half-inches, and eighths, and exposed in the first instance 

 to strong sunshine : thermometer 80° Fahr. ; atmosphere remark- 

 ably calm. , , ■ _, . . .....^a...i,, 



* Hence it Is that the strongest light appears to produce the deepest shadow. A total 

 eclipse of the sun occasions a more sensible darkness than midnight, being moreittiriie- 

 diately contrasted with the strong light of noon-day. . , . : 



f I should mention that in all these experiments the snufF wa$. carefully jemoY^ 

 with a sharp scissars, whenever a quarter of an inc"h of taper was consumed. This was 

 obviously necessary as the length of the suufF is known to ififluence materially the rate of 

 combustion. 



