1881.] of Radiant Heat upon Gaseous Matter. 



313 



All the results described in my first note have "been obtained in a 

 very energetic form with a battery of sixty Grove's cells. 



On the 4th of January I chose for my source of rays a powerful 

 lime-light, which, when sufficient care is taken to prevent the pitting 

 of the cylinder, works with admirable steadiness and without any 

 noise. I also changed my mirror for one of shorter focus, which 

 permitted a nearer approach to the source of rays. Tested with this 

 new reflector the stronger vapours rose remarkably in sounding 

 power. 



Improved manipulation was, I considered, sure to extract sounds 

 from rays of much more moderate intensity than those of the lime- 

 light. For this light, therefore, a common candle flame was sub- 

 stituted. Received and thrown back by the mirror, the radiant heat 

 of the candle produced audible tones in all the stronger vapours. 



Abandoning the mirror and bringing the candle close to the 

 rotating disk, its direct rays produced audible sounds. 



A red-hot coal, taken from the fire and held close to the rotating 

 disk, produced forcible sounds in a flask at the other side. 



A red-hot poker, placed in the position previously occupied by the 

 coal, produced strong sounds. Maintaining the flask in position 

 behind the rotating disk, amusing alternations of sound and silence 

 accompanied the alternate introduction and removal of the poker. 



The temperature of the iron was then lowered till its heat just 

 ceased to be visible. The intermittent invisible rays produced audible 

 sounds. 



The temperature was gradually lowered, being accompanied by a 

 gradual and continuous diminution of the sound. "When it ceased to 

 be audible the temperature of the poker was found to be below that 

 of boiling water. 



As might be expected from the foregoing experiments, an incandes- 

 cent platinum spiral, with or without the mirror, produced musical 

 sounds. When the battery power was reduced from ten cells to three, 

 the sounds, though enfeebled, were still distinct. 



My neglect of aqueous vapour had led me for a time astray in 1859, 

 but before publishing my results I had discovered my error. On the 

 present occasion this omnipresent substance had also to be reckoned 

 with. Fourteen flasks of various sizes, with their bottoms covered 

 with a little sulphuric acid, were closed with ordinary corks and per- 

 mitted to remain in the laboratory from the 23rd of December to the 

 4th of January . Tested on the latter day with the intermittent beam, 

 half of them emitted feeble sounds, but half were silent. The sounds 

 were undoubtedly due, not to dry air, but to traces of aqueous 

 vapour. 



An ordinary bottle, containing sulphuric acid for laboratory pur- 

 poses, being connected with the ear and placed in the intermittent 



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