liquids being transparent to light, the efficient rays which they intercepted 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 experiments 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 permitted 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 substituted. Received and thrown back by the mirror, the radiant heat of the candle produced audible tones in all the stronger vapors. 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-
