336 PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
The universality of its presence, and the discussions which it has provoked, rendered 
the action of water vapour especially interesting to me. I did not imagine at the 
outset that the modicum of vapour diffused in atmospheric air at ordinary tempera¬ 
tures could produce sonorous pulses of sensible intensity. In my first experiment, 
therefore, I warmed water in a flask nearly to its boiling point. I heated the flask 
above the water with the spirit lamp flame, thus dissipating every trace of haze, and 
then exposed the clear vapour to the intermittent beam. The experiment was a 
virtual question put to the vapour as to whether I was right or wrong in ascribing to 
it the power of absorbing radiant heat. The vapour answered by emitting a musical 
note which, when properly converged upon the tympanum, seemed as loud as the peal 
of an organ. When the temperature was lowered from 100° C. to 10° C., the sound 
did not vanish, as I expected it would. It remained not only distinct but strong. 
The flasks employed in these experiments were dried in a variety of ways, of which I 
have already given some account, and which will suggest themselves to every experi¬ 
menter in this field. Taken open from the laboratory, and exposed to the intermittent 
beam, the flasks are always to some extent sonorous. Placed beside sulphuric acid 
underneath the receiver of an air-pump, and permitted to dry there, they are reduced 
to silence. The slightest invasion of humid air renders them again sonorous. 
Breathing for a moment into a dried and silent flask, a loud sounding power is 
immediately manifested. 
Flasks without lips have been specially blown for these experiments, the india- 
rubber tubing being readily pushed over their necks. Large flasks are not the most 
suitable. To produce effective pulses sudden and intense expansions and contractions 
are required, and these are best obtained when the beam, at its place of maximum 
concentration, covers a large portion of the matter in the flask. Thin bulbs about a 
cubic inch in volume are both handy and effective ; but the bulb may be reduced to 
gT-th, or even y^th 0 f a cubic inch, without rendering the sound insensible. A speck of 
water introduced into such tiny bulbs, when vaporised by heat, produces sounds which 
are not only sensible but loud. A series of bulbs which I have actually employed in 
my experiments are represented in their natural dimensions in fig. 7. 
It cannot be necessary to state that the absorption which produces the pulses is 
direct and immediate, being the act of the gaseous molecules. The pulses are not due 
to the heating of the glass envelope and the communication of its heat to what it 
contains. For, were this the mode of heating, air would be as sonorous as olefiant gas. 
Nor are the pulses due to the sudden vaporization of a liquid layer which might be 
supposed to overspread the interior of the flask. When water at a low temperature is 
purposely caused to cover the interior surface, exposure to the beam produces sound of 
a certain intensity. When the flask is so heated in a spirit flame as to chase away 
every trace of the adherent liquid, the exposure of the pure vapour, then within the 
flask, to the beam, generates a sound far louder than that produced when the liquid 
film was there. Holding the bulb containing the hot vapour for a little time in the 
