ACTION OF RADIANT HEAT, 



39 



liquids being transparent to liglit, the efficient rays which they inter- 

 cepted must have been those of obscure heat. 



A layer of bisulphide of carbon, about ten times the thickness of 

 the transparent layers just referred to, and rendered opaque to light by 

 dissolved iodine, was interposed in the path of the intermittent beam. 

 It produced hardly any diminution of the sounds of the more active 

 vapors a further proof that it is the invisible heat-rays, to which the 

 solution of iodine is so eminently transparent, that are here effectual. 



Converting one of the small flasks used in the foregoing experi- 

 ments into a thermometer-bulb, and filling it with various gases in 

 succession, it was found that with those gases which yielded a feeble 

 sound the displacement of a thermometric column associated with the 

 bulb was slow and feeble, while with those gases which yielded loud 

 sounds the displacement was prompt and forcible. 



Since the handing in of the foregoing note, on the 3d of January, 

 the experiments have been pushed forward, augmented acquaintance 

 with the subject serving only to confirm my estimate of its interest 

 and importance. 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 per- 

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

 reflector the stronger vapors 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 substi- 

 tuted. Received and thrown back by the mirror, the radiant heat of 

 the candle produced audible tones in all the stronger vapors. Aban- 

 doning 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. Maintain- 

 ing the flask in position behind the rotating disk, amusing alternations 

 of sound and silence accompanied the alternate introduction and re- 

 moval 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 boilinor water. 



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



