Intelligence and Miscellaneous Articles. 309 



rock salt. In the middle between the two plates the tube is per- 

 forated in two diametrically opposite places. The upper opening 

 communicates with a small glass tube that can be closed by a cock ; 

 the lower with a somewhat longer glass tube, descending vertically, 

 which during the experiment dips in a vessel containing a coloured 

 fluid. The fluid stands some centims. higher in the tube than in 

 the vessel. 



In front of one of the rock-salt plates, in a line with the glass 

 tube, is a source of heat, say the gas-flame of an argand burner ; 

 between the flame and the tube a diaphragm about 4 centims. wide 

 and a double screen of sheet metal are placed : the latter can be 

 quickly drawn back and pushed forward. 



Now the experiment is made in the following manner: — The 

 height of the liquid in the manometer having been observed while 

 the screen shut out the heat-radiation, the screen is quickly with- 

 drawn ; by the absorption of rays which now takes place on the 

 part of the gas enclosed in the apparatus the gas is heated, in con- 

 sequence of which the manometer shows a sudden increase of pres- 

 sure, which, after some time, reaches a maximum. The increase, 

 especially that which takes place at the first moment, is very dif- 

 ferent with different gases — comparatively slight with air, while it 

 is considerable in the case of the strongly absorbing illuminating- 

 gas and ammonia. 



If the screen be then again pushed between the flame and the 

 glass tube, the pressure diminishes, in correspondence with the 

 cooling of the gas — -at first quickly, afterwards slowly. 



The phenomenon is, on the whole, tolerably complicated, because, 

 besides the absorption-capacity, the specific heat also, as well as the 

 ability of the gas to equalize more or less rapidly any differences of 

 temperature that may be present, play a part ; yet it is very suitable 

 for a demonstration-experiment. 



Now, after obtaining from M. Breguet's paper in the Journal de 

 Physique for November 1880 a knowledge of some details of Mr. 

 Graham Bell's experiments with the so-called photophone, the 

 question arose in my mind whether the gas enclosed in the glass 

 tube in the above-described experiment could not by intermittent 

 irradiation be caused to emit sounds. The above-mentioned 

 sudden rise and fall of the pressure at the moments when the 

 irradiation commences and ceases respectively permitted a fa- 

 vourable answer to the question to appear possible ; and experi- 

 ment has, in fact, confirmed my supposition in a very satisfactory 

 manner. 



As the source of heat, I used Drummond's lime-light. By two 

 lenses the rays were concentrated upon a notched disk of paste- 

 board, which could be rapidly rotated about a horizontal axis by 

 means of a cord. In order to deaden as much as possible the noise 

 arising from the rotation of the disk, it rotated between two larger 

 fixed disks, which were provided with a notch corresponding to 



Phil. Mag. S. 5. Vol. 11. No. 68. April 1881. Z 



