which made him averse from overloading his pages with technical experimental details.
The controversy above referred to I think we may now consider to be closed. Nobody now doubts the absorbing power of aqueous vapor. Indeed, the question seems to have entered upon a new phase; for in a recent number of Wiedemann's Annalen, Paschen investigates the precise position in the spectrum of the rays which are absorbed by aqueous vapor.
I can not attempt to show you here any of the early experiments on the absorption of vapors. But some years later Tyndall contrived an experiment, which will allow of reproduction. It is founded on some observations of Graham Bell, who discovered that various bodies became sonorous when exposed to intermittent radiation.
The radiation is supplied from incandescent lime, and is focused by a concave reflector. In the path of the rays is a revolving wheel provided with projecting teeth. When a tooth intervenes, the radiation is stopped; but in the interval between the teeth the radiation passes through, and falls upon any object held at the focus. The object in this case is a small glass bulb containing a few drops of ether, and communicating with the ear by a rubber tube. Under the operation of the intermittent radiation the ether vapor expands and contracts; in other words, a vibration is established, and a sound is heard by the observer. But if the vapor were absolutely diathermanous, no sound would be heard.
I have repeated the experiment of Tyndall which allowed him to distinguish between the behavior of ordinary air and dry air. If, dispensing with ether, we fill the bulb with air in the ordinary moist state, a sound is heard with perfect distinctness, but if we drop in a little sulphuric acid, so as to dry the air, the sound disappears.
According* to the law of exchanges, absorption is connected with radiation; so that while hydrogen and oxygen do not radiate, from ammonia we might expect to get considerable radiation. In the following experiment I aim at showing that the radiation of hot coal gas exceeds the radiation of equally hot air.
The face of the thermopile, protected by screens from the ball itself, is exposed to the radiation from the heated air which rises from a hot copper ball. The effect is manifested by the light reflected from a galvanometer mirror. When we replace the air by a stream of coal gas, the galvanometer indicates an augmentation of heat, so that we have before us a demonstration that coal gas when heated does radiate more than equally hot air, from which we conclude that it would exercise more absorption than air.