Scientific Lectures. 161 



nated by incandescent gas, and the light of these gas flames is pure 

 white light ; and yet we have a gas and not a solid burning. But 

 this is no exception ; for the light from these gas flames does not come 

 from the gas. It comes from minute particles of solid charcoal in the 

 gas. The gas consists of two parts ; the light gas we call hydrogen, 

 and the solid we call charcoal, or carbon. The hydrogen forms fii'st, 

 and in burning it scatters tlirough tlie whole mass of the flame a fine 

 charcoal dust, what we call soot ; and these particles of solid incan- 

 descent charcoal give the liglit. [Applause.] If we mix the gas 

 with the amount of air required to burn it, so that each solid particle 

 of charcoal shall be surrounded by its atmosphere with just the 

 amount requisite for burning, then the charcoal will burn with the 

 h3'drogen, and the result is that we shall have white light. If for 

 example, I simply open an orifice at the bottom of these burners, so 

 as to let in a certain amount of air to mix with the gas before it is 

 burned, then the white light disappears, and we have only the faint 

 blue light from the gas itself. These burners, which are generally 

 known under the name of Bunsen burners, give us a very ready means 

 for experimenting upon the light which emanates from different 

 incandescent vapors. Placing upon a platinum wire a bit of common 

 salt, I will insert it in this flame. The heat of the flame volatilizes 

 the salt, and the whole mass of the flame soon becomes filled with 

 salt vapor, which emits its peculiar light. The light which you now 

 see is of course a complex tint, because it is the light of the salt plus 

 the light of the gas ; but the light of the salt is so vastly more intense 

 than the light of the gas, that it wholly overpowers it. This light is 

 a perfectly pure monochromatic light. Salt is itself a compound 

 body. It is composed of a metal which we call sodium, and a ga& 

 which we call chlorine, both of which emit light ; but the light of the 

 Bodium is from a dense metallic vapor, and so much more intense than 

 the light from the chlorine gas that it wholly overpowers it ; and the 

 light you see is really from the sodium vapor, and is what we call the 

 sodium flame. [The lights in the hall were here turned down.] I 

 will now hold before this sodium flame what will appear to you as a 

 white card. Turning on the white light, you see that this is not a 

 white card at all, but is really red. [Applause.] There is no red 

 light in the sodium flame. The red color is in the light, and not in 

 the card ; and as this flame contained no red rays, we could not see 

 the red color. We must have white light in the flame in order to 

 see the red reflected from the card. If we point our spectroscope 

 [Inst.] 11 



