154 The Right Hon. Lord Rayleigh [May 7, 



by lateral leakage (scattering), which simulates the effect of absorp- 

 tion. He then found that the air itself, regarded as an assemblage 

 of small particles (molecules of oxygen and nitrogen), would have an 

 apparent absorbing power not much less than that actually deduced 

 by observations of the sun at different altitudes. The inference was 

 that the air itself was capable of accounting for much, if not all, of 

 the scattering which is observed in the blue sky; in fact that the 

 molecules of air are the small particles in question. 



When a beam of sunlight enters a room through a small aperture 

 in the shutter, its course is readily traced by the brightly illuminated 

 motes in the air. Prof. Tyndall, working in this Institution, devoted 

 much attention to the nature of these motes, and the methods by 

 which they may be got rid of. His results may be consulted in the 

 fascinating essay on " Floating Matter." One way of getting rid of 

 the motes is to filter the air through cotton wool. We have here 

 one of Tyndall's own experimental tubes. The electric beam passes 

 axially along it, and is concentrated to a focus about the middle of 

 the length. Its track is conspicuous. If now we displace the air 

 originally in the tube by filtered air, you see that the cone of light 

 fades into invisibility. 



Another of Tyndall's experiments was merely to place a spirit- 

 lamp or Bunsen burner under the beam. Since most of the dust 

 particles are combustible, the gases rising from the flame are free 

 from them. As you now see, dark rifts appear in the beam where the 

 uprising stream of dust-free gases traverses it. 



Tyndall, on the strength of these experiments, stated without 

 qualification that dust-free air does not scatter light, but my father's 

 views and theory lead clearly to the conclusion that it does. But 

 when I asked him what he thought about the feasibility of detecting 

 it by a laboratory experiment, he was not very sanguine of success. 

 It seemed worth while, however, to make the attempt, and I came to 

 the conclusion that the difficulty was not so much in the faintness of 

 the effect to be looked for as in the avoidance of stray light which 

 came into competition with it. The essential thing is to get a per- 

 fectly black background against which the beam (viewed transversely) 

 can be observed. We cannot get this with a vessel like Tyndall's 

 tube, just used. It is necessary to have what may be called a black 

 cave, and to view the beam as it crosses in front of the mouth of the 

 cave, the latter forming the background. If the cave is deep enough, 

 there is no limit to the blackness attainable. The great sensitiveness 

 of the well-rested eye, or the photographic plate, can then be brought 

 to bear, and the track of the beam can be well seen, however care- 

 fully the dust is removed. 



Some persons have been inclined to question whether the dust is 

 removed completely in these experiments. As a matter of fact this 

 is not where the difficulty lies at all. Dust so fine as to be 'very 

 difficult of filtration is an arm-chair conception, not encountered in 



