It is hardly necessary for me to say a word to guard myself against the misconception that I consider sound to be assisted by the fog itself. The fog-particles have no more influence upon the waves of sound than the suspended particles stirred up over the banks of Newfoundland have upon the waves of the Atlantic. A homogeneous air is the usual associate of fog, and hence the acoustic clearness of foggy weather.
§ 6. Experiments on Artificial Fogs
These observations are clinched and finished by being brought within the range of laboratory experiment. Here we shall learn incidentally a lesson as to the caution required from an experimenter.
The smoke from smouldering brown paper was allowed to stream upward through its rectangular apertures, into the tunnel a b c d (Fig. 146); the action upon the sound-waves was strong, rendering the short and agitated sensitive flame k tall and quiescent.
Air first passed through ammonia, then through hydrochloric acid, and, thus loaded with thick fumes, was sent into the tunnel; the agitated 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 upon the sound-waves was very strong.
The dense smoke of resin, burned before the open end of the tunnel, and blown into it with a pair of bellows, had also the effect of stopping the sound-waves, so as to still the agitated flame.
The conclusion seems clear, and its perfect harmony with the prevalent à priori notions as to the action of fog upon sound makes it almost irresistible. But caution is here necessary. 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 upper 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 produced 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 observed. 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.