1894.] on the Scientific Work of Tyndall. 223 



this subject. TyndalFs investigations, however, led him to favour 

 another explanation. His view was that sound was actually reflected 

 by atmospheric irregularities. He observed, what appears to bo 

 amply sufficient to establish his case, that prolonged signals from 

 fog sirens give rise to echoes audible after the signal has stopped. 

 This echo was heard from the air over the sea, and lasted in many 

 cases a long time, up to 15 seconds. There seems here no alternative 

 but to suppose that reflection must have occurred internally in the 

 atmosphere. In some cases the explanation of the occasional 

 diminished penetration of sound seems to be rather by refraction, 

 and in others by reflection. 



Tyndall proved that a single layer of hot air is sufficient to cause 

 reflection, and I propose to repeat his experiment. The source of 

 Bound, a toy reed, is placed at one end of one metallic tube, and a 

 sensitive flame at one end of a second. The opposite ends of these 

 tubes are placed near each other, but in a position which does not 

 permit the sound waves issuing from the one to enter the other 

 directly. Accordingly the flame shows no response. If, however, a 

 pane of glass be held suitably, the waves are reflected back and the 

 flame is excited. Tyndall's experiment consists in the demonstra- 

 tion that a flat gas flame is competent to act the part of a reflector. 

 When I hold the gas flame in the proper position, the percipient 

 flame flares ; when the flat flame is removed or held at an unsuit- 

 able angle, there is almost complete recovery. 



It is true that in the atmosphere no such violent transitions of 

 density can occur as are met with in a flame ; but, on the other hand, 

 the interruptions may be very numerous, as is indeed rendered 

 probable by the phenomena of stellar scintillation. 



The third portion of my subject must be treated very briefly. 

 The guiding idea of much of Tyndall's work on atmospheric particles 

 was the application of an intense illumination to render them evident. 

 Fine particles of mastic, precipitated on admixture of varnish with a 

 large quantity of water, had already been examined by Briicke. Che- 

 mically precipitated sulphur is convenient, and allows the influence 

 of size to be watched as the particles grow. But the most interesting 

 observations of Tyndall relate to precipitates in gases caused by the 

 chemical action of the light itself. This may be illustrated by causing 

 the concentrated rays of the electric lamp to pass through a flask 

 containing vapour of peroxide of chlorine. Within a few seconds 

 dense clouds are produced. 



When the particles are very small in comparison with the wave 

 length, the laws governing the dispersion of the light are simple. 

 Tyndall pursued the investigation to the case where the particles have 

 grown beyond the limit above indicated, and found that the polarisa- 

 tion of the dispersed light was effected in a peculiar and interesting 

 manner. 



Atmospheric dust, especially in London, is largely organic. If, 



