394 Professor Tyndall [June 7, 



modes of motion ; and the vibration of a ray claims strict brotherhood 

 with the vibrations of our pendulum. Suppose ethereal waves striking 

 upon atoms which oscillate in the same periods as the waves succeed 

 each other, the motion of the waves will be absorbed by the atoms ; 

 suppose we send our beam of white light through a sodium flame, the 

 particles of that flame will be chiefly affected by those undulations 

 which are synchronous with their own periods of vibration. There will 

 be on the part of those particular rays a transference of motion from 

 the agitated ether to the atoms of the volatilized sodium, which, as 

 already defined, is absorption. We use glass screens to defend us 

 from the heat of our fires ; how do they act ? Thus : — The heat ema- 

 nating from the fire is for the most part due to radiations which are 

 incompetent to excite the sense of vision ; we call these rays obscure. 

 Glass, though pervious to the luminous rays, is opaque in a high degree 

 to those obscure rays, and cuts them off", while the cheerful light of the 

 fire is allowed to pass. Now mark me clearly. The heat cut off" from 

 your person is to be found in the glass, the latter becomes heated and 

 radiates towards your person ; what then is the use of the glass if it 

 merely thus acts as a temporary halting-place for the rays, and sends 

 them on afterwards. It does this: — It not only sends the heat it 

 receives towards you, but scatters it also in all other directions round 

 the room. Thus the rays which, were the glass not interposed, would 

 be shot directly against your person, are for the most part diverted 

 from their original direction, and you are preserved from their impact. 



Now for our experiment. I pass the beam from the electric lamp 

 through the two prisms, and the spectrum spreads its colours upon the 

 screen. Between the lamp and the prism I interpose this snapdragon 

 light. Alcohol and water are here mixed up with a quantity of common 

 salt, and the metal dish that contains them is heated by a spirit-lamp. 

 The vapour from the mixture ignites and we have this monochromatic 

 flame. Through this flame the beam from the lamp is now passing; 

 and observe the result upon the spectrum. You see a dark band cut 

 out of the yellow, — not very dark, but sufficiently so to be seen by 

 everybody present. Observe how the band quivers and varies in shade 

 as the amount of yellow light cut off" by the unsteady flame varies in 

 amount. The flame of this monochromatic lamp is at the present 

 moment casting its proper yellow light upon that shaded line ; and 

 more than this, it casts, in part, the light which it absorbs from the 

 electric lamp upon it ; but it scatters the greater portion of this light 

 in other directions, and thus withdraws it from its place upon the 

 screen, as the glass, in the case above supposed, diverted the heat of 

 the fire from your person. Hence the band appears dark ; not abso- 

 lutely, but dark in comparison with the adjacent brilliant portions of 

 the spectrum. 



But let me exalt this effect. I place in front of the electric lamp 

 the intense flame of a large Bunsen's burner. I have here a platinum 

 capsule into which I put a bit of sodium less than a pea in magnitude. 

 The sodium placed in the flame soon volatilizes and burns with bril- 



