termittent beam. The well-dried air within the bulbs proved silent, while the slightest admixture of humid air sufficed to endow it with sounding power. Placing a little water in a thin glass bulb, and heating it nearly to its boiling-point, the sounds produced by the developed vapor are exceedingly loud. The bulbs employed in these experiments are usually about a cubic inch in volume. They may, however, be reduced to one fiftieth or even one one-hundredth of a cubic inch. When a minute drop of water is vaporized within such little bulbs, on their exposure to the intermittent beam loud musical sounds are produced.
It is to be borne in mind that the heat employed in these experiments, coming as it did from a highly luminous source, was absorbed in a far smaller degree than would be the heat from bodies under the temperature of incandescence.
To render the correlation of sound-producing power and athermancy complete, all the gases and vapors which had been exposed to the intermittent beam were examined as to the augmentation of their elastic force through the absorption of radiant heat. A glass cylinder, four inches long and three inches in diameter, had its ends closed with transparent plates of rock-salt. Connected with this cylinder was a narrow U-tube, containing a colored liquid which stood at the same level in the two arms of the U. The cylinder could be exhausted at pleasure or filled with a gas or vapor. When filled, the sudden removal of a double silvered screen permitted the beam from the lime-light to pass through it, the augmentation of elastic force being immediately declared by the depression of the liquid in one of the arms of the U-tube and its elevation in the other. The difference of level in the two arms gave, in terms of water-pressure, a measure of the heat absorbed. With the stronger vapors it would be easy with this instrument to produce an augmentation of elastic force corresponding to a water-pressure of a thousand millimetres. As might be expected, the intensity of the sounds corresponded with the energy of the absorption, varying from "exceedingly strong," "very strong," "strong," "moderate," "weak," to "inaudible." In this connection reference was made to the interesting experiments of Professor Röntgen, an independent and successful worker in this field.
In conclusion, the lecture draws attention to the bearing of its' results upon the phenomena of meteorology. The views of Magnus, regarding the part played by mist or haze, are referred to, and attention is directed to various observations by Wells, which are in opposition to these views. The observations of Wilson, Six, Leslie, Denham, Hooker, Livingstone, Mitchell, Strachey, and others are referred to and connected with the action of aqueous vapor upon solar and terrestrial radiation. Many years ago the lecturer sought to imitate the action of aqueous vapor on the solar rays by sending a beam from the electric light through a layer of water, and afterward examining its spectrum.
