olO Intelligence and Miscellaneous Articles. 



each of the openings in the rotating disk and closed by a thin plate 

 of glass. 



Behind these notches the absorption -apparatus was either fixed 

 or held free in the hand ; in this experiment it had a length of 12 

 centims.; the manometer was replaced by a short glass tube of 

 1 centim. diameter, over which was pushed a wide caoutchouc tube 

 that led to the ear of the observer and was inserted in it as deeply 

 as possible. 



The rays penetrated into the absorption-apparatus every time an 

 opening in the rotating disk came in front of the rock-salt plate ; 

 the interruption of this was effected by the unnotched parts of the 

 disk. 



At the commencement the apparatus was filled with air ; on rota- 

 ting the disk I could not perceive any tone, perhaps because, on 

 account of the rotation &c, too many extraneous noises were still 

 present. The affair, however, took quite another shape when 

 illuminating-gas was substituted for air ; the tone could be heard 

 with extraordinary distinctness, and might almost be compared to 

 the whistling of a not too strong wind. Its height varied with the 

 velocity of the rotation ; and the tone vanished only when the rota- 

 tion became very rapid. The strength of the tone varied percep- 

 tibly with the time during which the cube was exposed ; but the 

 tones ceased directly if the rays were intercepted by an impervious 

 body (as the hand, a small wooden board, or a hardgum plate) held 

 before the disk. 



"With ammonia gas I likewise obtained distinct tones ; while dry 

 hydrogen and oxygen behaved like atmospheric air. 



The explanation of these experiments is not far to seek ; it has 

 already been intimated above. We have not to do with any new 

 property of the rays ; the heating and expansion produced by ab- 

 sorption, and the subsequent cooling and contraction of the absorb- 

 ing body, are the causes of the phenomena. That the gas really 

 played the chief part in my experiments, and not the glass, upon 

 which likewise the rays fell, follows even from this — that only the 

 strongly absorbent gases emit audible tones ; and I found direct proof 

 of it by so directing the rays, in some experiments, by means of 

 a third lens and a diaphragm, that they passed merely through the 

 rock salt and the gas, without anywhere coming into contact with 

 the glass wall of the tube : the effect was substantially the same 

 ft with the simple irradiation. 



A solution of alum placed in the path of the rays caused an 

 mstantaneous cessation of the tone; on the contrary, scarcely a 

 weakening could be observed when the rays had passed through a 

 layer 10 centims. in thickness of solution of iodine (in sulphide of 

 carbon). Consequently it is the less-refrangible rays which are 

 most operative, at least upon illuminating-gas and ammonia. 



I purpose investigating the behaviour of aqueous vapour, in the 

 hope of furnishing in this way a contribution to the decision of the 

 question whether it to any considerable extent absorbs heat-rays or 



