Scientific Lectures. 189 



but denser than our atmosphere. When the bottle is uncorked, I can 

 pour out this dense air, and you notice, on the screen, how the diver- 

 gent beams of light render it visible ; we see it streaming out, floating 

 away, and can follow the wavy fluctuations it is subjected to by cur- 

 rents of air. If I had used the hot, rarer air which streams up from 

 a candle flame, it would have been equally visible, at a distance of 

 many inches above the flame itself. 



In our experiment we plainly saw the layers which were denser 

 than the atmosphere, by the aid of deviation of the light which they 

 caused; and it is very evident that if we could only isolate just the 

 particular rays concerned, getting thus rid of the overpowering glare 

 of the unaffected light, the whole phenomenon would have been far 

 more distinct. Toepler contrived to do this very thing, by the aid 

 of a peculiar screen, and thus was able to deal with the far more subtile 

 variations in density involved in the case of a sound-wave. 



But a steady light, like the one we have employed, would have 

 been of no use in such an experiment ; the sound-wave travels as fast 

 as a cannon-ball, and a light of this character would render visible 

 neither one nor the other. It should be illuminated by an instanta- 

 neous flash of light, so that the wave would not have a chance to 

 move perceptibly while lighted up, and for this purpose the electric 

 spark was just what was wanted. Let me make an experiment to 

 illustrate this point. We have before us a disc six feet in diameter, 

 which is being turned so rapidly that you cannot see the design painted 

 on it ; the disc looks merely gray. The gas-lights having been turned 

 down, I illuminate our swiftly spinning disc, from time to time, with 

 the electric spark ; it seems to stand still, and you readily see that it 

 actually is painted in great black and white sectors. This talented 

 young German physicist used then the electric spark to illuminate his 

 soundrwaves, and the snap of a second electric spark to generate them. 



We have now upon the screen the greatly enlarged representation 

 of one of these waves, as seen by Toepler's method ; it is spherical, as 

 we expected, and beautifully shaded. 



Still more remarkable is the circumstance that this physicist also 

 succeeded in observing the reflection of a sound-wave against a solid 

 body. My copy of one of his drawings is on the screen, and you can 

 see that the reflected portion is likewise spherical. Finally, after 

 many efforts, he was so fortunate as to be able repeatedly to observe 

 the transmission of a sound-wave from a dense to a rare gas, that is, 

 to see a sound-wave refracted. This also is now before you on the 

 screen ; the wave leaves the air, and is entering hydrogen gas, and 



