416 MR WILLIAM SWAN ON THE PRISMATIC SPECTRA OF THE 



light of every degree of refrangibility between limits varying with its temperature. 

 Thus, when carbon burns in oxygen, or when a strip of platinum is heated, to a 

 temperature of 2130° Fahr., by the passage of a current of electricity, a perfectly 

 continuous spectrum is produced, without any bright lines or dark spaces, and ex- 

 tending at least from the line B to the line H of the solar spectrum.* This enables 

 us to explain why we may see, in the spectrum formed by the blue portion of a 

 flame, bright lines and dark spaces, which are totally invisible in the spectrum of 

 the bright inner cone. For the light of the inner cone arises from incandescent solid 

 carbon deposited in the flame, which, as we have just now seen, must produce a 

 brilliant continuous spectrum. The light of this spectrum overpowers the compa- 

 ratively faint illumination of the bright lines of the spectrum formed by the blue 

 part of the flame, while it fills up the dark intervals between them ; and both 

 causes conspiring render the lines invisible. Again, lines may easily be seen in 

 the spectrum of the blue part of a spirit lamp or candle flame, which fail to show 

 themselves when we examine the flames of oil of turpentine or coal naphtha ; for 

 the latter bodies contain so much carbon that it begins to be deposited almost at 

 the very bottom of the flame. The blue conoid is thus reduced to an extremely 

 narrow ring ; and it is practically impossible, however small the aperture through 

 which light passes to the prism, to obtain the spectrum of the blue light separated 

 from that of the incandescent carbon. 



We can also similarly explain why lines may be visible in the spectrum of 

 alcohol, which may not be easily seen in that of weak spirit of wine, or of im- 

 pure wood spirit. The exterior envelope, like the interior bright cone, derives 

 most of its light from incandescent solid matter, and produces a continuous spec- 

 trum, as was shown in the case of the Bunsen lamp. Now the exterior envelope 

 of the flame of weak spirit of wine, or of wood spirit, is very voluminous and 

 fully developed, and hence of unusual thickness near the bottom of the flame. 

 The light derived from the incandescent matter it contains, will therefore operate 

 precisely like that of the interior luminous cone, in rendering the bright lines of 

 the spectrum invisible. 



On Methods of Observing the Spectra of Carbohydrogen Flames. 



Since the continuous spectra due to the light of incandescent matter offer 

 no distinguishing features, it follows, that in searching for phenomena charac- 

 teristic of the chemical constitution of bodies undergoing combustion, we must 

 examine that part of the flame in which any solid molecules, which are being de- 

 posited, have not been able to collect into masses. This state of things exists in 

 the blue portion of the carbohydrogen flames, where the supply of air is sufficient 

 completely to consume the gases. In flames, such as that of oil of turpentine, 

 C 10 H 8 , where there is much carbon, it becomes necessary to burn the carbon by an 



* Phil. Mag., vol. xxx., p. 349. 



