334 
PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
rubber tube, ending in a hollow cone of ivory or boxwood, connected the flask with the 
ear. With this arrangement, simple as it is, sounds of surprising intensity were 
obtained with all those gases and vapours which my previous experiments with the 
experimental tube and thermopile had proved to be powerful absorbers of radiant 
heat. The final arrangement was that shown in fig. 6. 
Fig. 6. 
The source of heat is the carefully worked and centred lime-cylinder L, heated by 
the oxy-hydrogen flame. The rays from this source are received by the concave 
mirror It, and converged upon the bulb B which contains the substance 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 disc D of strong cardboard, 2 feet in diameter, and provided at the circum¬ 
ference with 29 teeth and corresponding interspaces.* The disc 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 upside 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 
* Intermittence is sometimes produced by the series of equidistant circular apertures shown in 
the figure. 
