Volume 2

Fragments of science : a series of detached essays, addresses, and reviews / by John Tyndall.

  • John Tyndall
Date:
1879
Catalogue details

Licence: Public Domain Mark

Credit: Fragments of science : a series of detached essays, addresses, and reviews / by John Tyndall. Source: Wellcome Collection.

    461/508 (page 449)
    wire when it first feels warm to the touch—wlien, therefore, all its rays are invisible—by the number 1, the invisible radiation from the same wire raised to a white heat may be 500 or more.' It is not, then, by the diminution or transformation of the nonJuminous emission that we obtain the luminous ■; the heat rays maintain their ground as the necessary antecedents and companions of the light rays. Wlien detached and concentrated, these powerful heat rays can produce all the effects ascribed to the mirrors of Ai'chimedes at the siege of Syracuse. While incompetent to produce the faintest glimmer of light, or to affect the most delicate air-thermometer, they will inflame paper, burn up wood, and even ignite combustible metals. When they im- pinge upon a metal refractory enough to bear their shock without fusion, they can raise it to a heat so while and luminous as to yield, when analysed, all t^e colours of the spectrum. In this way the dark rays emitted by the incandescent carbons are converted into light rays of all colours. Still, so powerless are these invisible rays to excite vision, that the eye has been placed at a focus competent to raise platinum foil to bright redness, without experiencing any visual im- pression. Light for light, no doubt, the amount of heat imparted by the incandescent carbons to the air is far less than that imparted by gas flames. It is less, because of the smaller size of the carbons, and of the comparative smallness of the quantity of fuel consumed in a given time. It is also less because the air cannot penetrate the carbons as it penetrates a flame. The temperature of the flame is lowered by the admixture of a gas which constitutes four-fifths of our atmosphere, and which, while it appropriates and diffuses the heat] does not aid in the combustion; and this lowering of ' See article 'Eadiatiou,' vol. i. VOL. II. G a