696 THE POPULAR SCIENCE MONTHLY. 



and thus loaded with thick fumes, was sent into the tunnel; the agi- 

 tated flame was rendered immediately quiescent, indicating a very 

 decided action on the part of the artificial fog. 



Air passed through perchloride of tin and sent into the tunnel 

 produced exceedingly dense fumes. The action of the fog upon the 

 sound-waves was very strong. 



The dense smoke of resin, burnt before the open end of the tunnel 

 and blown into it with a pair of bellows, had also the effect of stop- 

 ping the sound-waves, so as to still the agitated flame. 



The result seems clear ; and it perfectly harmonizes with the prev- 

 alent a priori notions as to the action of fog upon sound. But caution 

 is here necessary; for the smoke of the brown paper was hot; the 

 flask containing the hydrochloric acid was hot; that containing the 

 perchloride of tin was hot; while the resin-fumes produced by a red- 

 hot poker were also obviously hot. Were the results, then, due to the 

 fumes or to the differences of temperature? The observations might 

 well have proved a trap to an incautious reasoner. 



Instead of the smoke and heated air, the heated air alone from four 

 red-hot pokers was permitted to stream upward into the tunnel ; the 

 action on the sound-waves was very decided, though the tunnel was 

 optically empty. The flame of a candle was placed at the tunnel-end, 

 and the hot air just above its tip was blown into the tunnel; the 

 action on the sensitive flame was decided. A similar effect was pro- 

 duced when the air, ascending from a red-hot iron, was blown into 

 the tunnel. 



In these latter cases the tunnel remained optically clear, while the 

 same effect as that produced by the resin smoke and fumes was ob- 

 served. Clearly, then, we are not entitled to ascribe, without further 

 investigation, to the artificial fog an effect which may have been due 

 to the air which accompanied it. 



Having eliminated the fog and proved the non-homogeneous air 

 effective, our reasoning will be completed by eliminating the heat, and 

 proving the fog ineffective. 



Instead of the tunnel abed {see p. 689), a cupboard with glass sides, 

 three feet long, two feet wide, and about five feet high, was filled with 

 fumes of various kinds. Here it was thought the fumes might remain 

 long enough for differences of temperature to disappear. Two aper- 

 tures were made in two opposite panes of glass three feet asunder ; 

 in front of one aperture was placed the bell in its padded box and 

 behind the other aperture, and at some distance from it, the sensitive 

 flame. 



Phosphorus placed in a cup floating on water was ignited within 

 the closed cupboard. The fumes were so dense that considerably less 

 than the three feet traversed by the sound extinguished totally a bright 

 candle-flame. At first there was a slight action upon the sound ; but 

 this rapidly vanished, the flame being affected exactly as if the Bound 



