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. 22. GEISSLER’S EXPERIMENT. LUMINOSITY OF VACUUM TUBES BY FRICTION. INCREASED BY LOW TEMPERATURE. _Science Record_, 1873.—By rubbing the vacuum tubes with an insulator—cat skin, silk, etc.—he observed that light was generated and that its color depended upon the particular gas forming the residual atmosphere. At a low temperature, the colors were more luminous. § 135. The best form of tube consisted of a spiral tube contained within another tube. A modified construction involved the introduction of mercury. By exhausting the air, and shaking the tube, the friction or motion of the mercury against the glass produced luminous effects according to the gas. Only chemically pure mercury would cause the light, which endured for an instant after the rubbing ceased. § 63. 23. ALVERGNIAT’S EXPERIMENT. LUMINOSITY OF VACUUM TUBES BY FRICTION AND DISCHARGES. DIFFERENT VACUA REQUIRED. _Sci. Rec._, 1873, p. 111. _Comptes Rendus_, 1873.—To obtain luminosity by charging the tubes with the coil, it was necessary to increase the degree of the vacuum—but when this was done the rubbing of the tube would not cause light. The tube employed was 45 cm. in length, and contained a small quantity of silicic bromide. The atmospheric pressure within the tube for obtaining the glimmer by friction was 15 mm.