Page:Popular Science Monthly Volume 20.djvu/670

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By Professor JOHN TYNDALL.

THE Bakerian lecture of 1881 before the Royal Society opens with a brief reference to the researches of Leslie, Rumford, and Melloni. The labors of Tyndall and Magnus, as far as they bear upon the present subject, are then succinctly sketched, their points of difference being signalized and briefly discussed. This preliminary sketch is wound up by a reference to a recently published paper by Lecher and Pernter, who, while supporting the lecturer in the matter of gases, dissent from him in the matter of vapors. These investigators are especially emphatic in affirming the neutrality of aqueous vapor to radiant heat. Following Magnus, they refer Tyndall's results to vapor-hesion, that is to say, to the condensation of the vapors on the surfaces of the plates of rock-salt used to close the experimental tube, and on the interior surface of the tube itself.

In November, 1880, the lecturer's investigations in this field were resumed. Former experiments were repeated and verified with divers sources of heat, and with experimental tubes, some polished within, and others coated inside with lamp-black. The results obtained with the one class of tubes are substantially the same as those obtained with the other.

But even a coating of lamp-black may be supposed to reflect a certain amount of heat, hence the desirability of an arrangement whereby internal reflection should be entirely abolished. This was accomplished in the following manner: A spiral of platinum wire, rendered incandescent by a voltaic current of measured strength, was chosen as source of heat. An experimental tube thirty-eight inches long and six inches in diameter had two circular apertures at its ends, closed by transparent plates of rock-salt, three inches in diameter. The tube was furnished with three cocks—one connected with a Bianchi's large air-pump, another with a purifying apparatus, while through the third vapors and gases could be admitted. Prior to entering the tube, the calorific rays were sent through a very perfect rock-salt lens, by means of which an image of the spiral was formed on the most distant plate of rock-salt. To obtain the image with clearness, the spiral was first rendered highly luminous, and afterward reduced, by the introduction of resistance, to the required temperature. In this way a calorific beam was sent along the axis of the experimental tube without at all impinging upon its interior surface. No reflection came into play; no absorption by hypothetical liquid films, coating the internal surface,

  1. Abstract of the Bakerian Lecture of 1881, delivered by Dr. Tyndall, F.R.S., on the "Action of Free Molecules on Radiant Heat, and its Conversion thereby into Sound."