506 Prof. Tyndall on the Action of Free Molecules on 



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 3^, or even yoo> °^ a cu bic inch 

 without rendering the sound insensible. A speck of water 

 introduced into such tiny bulbs, Avhen vaporized by heat, 

 produces sounds which are not only sensible, but loud. A 

 series of bulbs which I have actually employed in my experi- 

 ments 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 



