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IV.

20. Boltzmann, Gibson, Barclay, Hopkinson and Gladstone’s Experiments. Square Root of the Dielectric Capacity Equal to the Refractive Index. Phil. Trans., 1871, p. 573. Maxwell, Vol. II., § 788. Maxwell has argued elaborately upon results of some of the above experimenters upon the theory that the luminiferous ether is the medium for transmission of electricity, light and magnetism; therefore he predicted that the relation stated in the title above should exist. He acknowledged that the relation is sufficiently near a constant to show in connection with other results, especially those obtained, that his theory is probably correct.

21. PLÜCKER’s Experiment. Hermetically Sealed Vacuum Tube. Encycl. Brit., vol. 8, p. 64. Pogg. Ann., 1858, and vol. CXXXVI, 1869.—He engaged Geissler (according to Hittorf) to make a glass tube in which the platinum wire electrodes were sealed in the glass by fusion, as in the modern incandescent lamp. After the air was exhausted by a mechanical air pump through a capillary tube, the same was sealed with the flame of a spirit lamp. He thus established means whereby a practically permanent vacuum could be maintained within a glass bulb. Platinum expands by heat at about the same rate as glass: hence there is no tendency to crack and admit air.