666 THE POPULAR SCIENCE MONTHLY. 



of the frequent failure of sound signals was tliat adopted by Prof. 

 Henry in America, a distinguished worker upon this subject. 

 Tyndall's investigations, however, led him to favor another ex- 

 planation. His view was that sound was actually reflected by 

 atmospheric irregularities. He observed, what appears to be 

 amply suflBcient 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 fifteen 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 sufiBcient to 

 cause reflection, and I propose to repeat his experiment. The 

 source of sound, 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 re- 

 sponse. 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 demonstration 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 re- 

 moved or held at an unsuitable 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 ren- 

 dered 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 admix- 

 ture of varnish with a large quantity of water, had already been 

 examined by Briicke. Chemically precipitated sulphur is conven- 

 ient, 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 vapor of 

 peroxide of chlorine. Within a few seconds dense clouds are pro- 

 duced. 



When the particles are very small in comparison with the 



