504 Prof. Tyndall on the Action of Free Molecules on 



to be tested. The bulb is connected with the ear by a piece 

 of india-rubber tubing, ending in a tapering tube of boxwood 

 or ivory. The intermittence of the calorific beam is effected 

 by the disk D of strong cardboard, 2 feet in diameter, and 

 provided at the circumference with 29 teeth and corresponding 

 interspaces*. The disk is caused to rotate by the wheel W, 

 with which it is connected by a band. The positions of the 

 sonorous bulb and of its ear-tube are shown in the figure. 

 In the case of gases lighter than air, the bulb B is turned up- 

 side down. With the heavier gases it is held erect. When 

 vapours are tested, a small quantity of liquid is poured into 

 the bulb, which is shaken so as to diffuse the vapour in the 

 air above the liquid. The bulb is held so that the point of 

 maximum concentration of the beam falls upon it. 



With this apparatus I have tested more than once the 

 sounding-power of ten gases and of about eighty vapours. 

 As a sound-producer chloride of methyl is supreme. It is, 

 however, closely followed by aldehyde, olefiant gas, and sul- 

 phuric ether, the two latter being very nearly equal to each 

 other. The volatility of the liquid from which the vapour is 

 derived is of course an important factor in the result. For 

 however high the inherent capacity of the molecule as an 

 absorber may be, if the molecules be scanty in number the 

 effect is small. Feeble vapours, on the other hand, may to 

 some extent atone by quantity for the inherent weakness of 

 their molecules. A few examples will suffice to show how 

 the specific action of the molecules overrides the effect of 

 volatility. Bisulphide of carbon, with a boiling-point of 

 43° C, is less powerful than acetic ether, with a boiling-point 

 of 74°. Tetrachloride of carbon boils at 77°; but its sound by 

 no means equals that of acetal, which boils at 104°. Chloro- 

 form, with a boiling-point of 61°, is less powerful as asound- 

 producer than valeral, with a boiling-point of 100°, or even 

 than valerianic ether, with a boiling-point of 144°. Cyanide 

 of methyl boils at 82° — but produces less sound than acetate 

 of propyl, with a boiling-point of 102°. In the experimental 

 tube these vapours follow, as absorbers, the order of their 

 sounds. When tested in liquid layers they follow the same 

 order. I have examined about a score of liquids with boiling- 

 points varying from 163° to 308°. At ordinary temperatures 

 the vapours of these liquids were practically inaudible; the 

 liquids being plunged in a bath of heated oil the vapours so 

 produced emitted, for the most part, powerful sounds. The 



* Intermittence is sometimes produced by the series of equidistant cir- 

 cular apertures shown in the figure. 



