314 Prof. Tyndall. Action of an Intermittent Beam [Jan. 13, 



beam, emitted a faint, but distinct, musical sound. This bottle had 

 been opened two or three times during the day, its dryness being thus 

 vitiated by the mixture of a small quantity of common air. A second 

 similar bottle, in which sulphuric acid had stood undisturbed for some 

 days, was placed in the beam : the dry air above the liquid proved 

 absolutely silent. 



On the evening of January the 7th Professor Dewar handed me four 

 flasks treated in the following manner. Into one was poured a small 

 quantity of strong sulphuric acid; into another a small quantity of 

 Nordhausen sulphuric acid ; in a third were placed some fragments of 

 fused chloride of calcium ; while the fourth contained a small quantity of 

 phosphoric anhydride. They were closed with well fitting india-rubber 

 stoppers, and permitted to remain undisturbed throughout the night. 

 Tested after twelve hours, each of them emitted a feeble sound, the 

 flask last mentioned being the strongest. Tested again six hours later 

 the sound had disappeared from three of the flasks, that containing the 

 phosphoric anhydride alone remaining musical. 



Breathing into a flask partially filled with sulphuric acid instantly 

 restores the sounding power, which continues for a considerable time. 

 The wetting of the interior surface of the flask with the sulphuric 

 acid always enfeebles, and sometimes destroys the sound. 



A bulb, less than a cubic inch in volume, and containing a little 

 water, lowered to the temperature of melting ice, produces very 

 distinct sounds. Warming the water in the flame of a spirit lamp, 

 the sound becomes greatly augmented in strength. At the boiling 

 temperature the sound emitted by this small bulb* is of extraordinary 

 intensity. 



These results are in accord with those obtained by me nearly 

 nineteen years ago, both in reference to air and to aqueous vapour. 

 They are in utter disaccord with those obtained by other experi- 

 menters, who have ascribed a high absorption to air and none to 

 aqueous vapour. 



The action of aqueous vapour being thus revealed, the necessity of 

 thoroughly drying the flasks, when testing other substances, becomes 

 obvious. The following plan has been found effective. Each flask is 

 first heated in the flame of a spirit-lamp till every visible trace of in- 

 ternal moisture has disappeared, and it is afterwards raised to a tem- 

 perature of about 400° C. While the flask is still hot, a glass tube is 

 introduced into it, and air freed from carbonic acid by caustic potash, 

 and from aqueous vapour by sulphuric acid, is urged through the flask 

 until it is cool. Connected with the ear-tube, and exposed immediately 

 to the intermittent beam, the attention of the ear, if I may use the 



* In such bulbs even bisulphide of carbon vapour may be so nursed as to produce 

 sounds of considerable strength. 



