322 FOURTH REPORT — 1834. 



It is easy to account for the general fact of the deficiency of 

 certain classes of rat/s in certain lights. When a body violently 

 heated begins to shine, the phenomenon is simply accounted 

 for, in the wave-theory, by an increase in the frequency of its 

 vibrations. In the same manner it seems natural to suppose, 

 generally, that the mechanical agencies at work during com- 

 bustion accelerate or retard, in various ways, the rate of vibra- 

 tion, and so alter the character of the emitted lights. The 

 light emitted in weak or incipient combustion is generally blue. 

 Sir John Herschel observed that when sulphur burns with a 

 feeble flame, its light contains all the rays of the spectrum, 

 and particularly the blue and violet ; while, in vivid combustion, 

 these disappear entirely, and the light is a yellow of almost 

 perfect homogeneity *. The various shades of colour in the 

 flame of a common candle, — from the deep blue of the lower 

 part (which is found by prismatic analysis to consist of five 

 distinct portions,) to the yellowish white in the centre, and 

 thence to the dusky red at the apex of the flame, — seem to be 

 referrible to the same principle. Fraunhofer and Sir David 

 Brewster have both remarked that the flame of oil, urged by 

 the blowpipe, consists chiehy or wholly of yellow rays. The 

 same fact was long since observed by Mr. Melville with respect 

 to the flame of alcohol, into which nitre, muriate of soda and 

 other salts had been introduced f ; and Sir David Brewster has 

 found that the quantity of yellow light given out by burning 

 bodies increases with their humidity, the flame of alcohol diluted 

 with water being nearly a homogeneous yellow;}:. It is more im- 

 portant to remark however, in illustration of the undulatory 

 view of the phenomenon of emission, that the colour of flames is 

 often found to depend on the presence of something which is it- 

 self unaltered in the process of combustion. Thus Mr. Talbot 

 has remarked that when a smaU quantity of muriate of lime was 

 placed on the wick of a spirit lamp, it gave out red and green 

 rays during an entire evening, though the salt was not sensibly 

 diminished §. The absence of definite rays in certain lights, 

 and the fixed lines of the solar spectrum, have been referred 

 by Sir John Herschel to the same principle by which he has 

 explained the absorption of specific rays || . 



In what has preceded we have assumed the truth of the re- 

 ceived theory with respect to the composition of solar light, 

 and the connexion between the colour of a ray and its refrangi- 

 bility. This theory however has been recently opposed by Sir 



* " On Absorption of Light in coloured Media," Edin. Trans., vol. ix. 

 i Ed'nib. Essays. J On a Monochromatic Lamp. Ihid. 



§ Ediub. Jaitni. of Science, v. 77. || Phil, il/rti/., Third Series, vol. iii. p. 407. 



