Lu minosity of Bare Earths when heated by Cathode Bays. 115 



" On the Luminosity of the Eare Earths when heated in Vacuo by 

 means of Cathode Bays." By A. A. Campbell Swinton. 

 Communicated by Lord Kelvin, RE.S. Eeceived March 20, 

 — Eead April 27, 1899. 



For incandescent gas mantles, it is found that certain definite mix- 

 tures of the rare earths are necessary, in order to obtain the maximum 

 luminosity. For instance, in the ordinary Bunsen gas flame, a mantle 

 consisting of pure thorium oxide, or of pure cerium oxide, will only 

 give about one-eleventh of the light that is given by a mantle com- 

 posed of 99 per cent, of thorium oxide, and 1 per cent, of cerium 

 oxide, which is the mixture at present used by the Welsbach Com- 

 pany. 



In order to explain this remarkable fact, several different and some- 

 what contradictory theories have been propounded, one of which implies 

 catalytic or other - chemical action between the oxides and the consti- 

 tuents of the Bunsen flame. 



In order to investigate this question, it is obviously important to 

 note the behaviour of the rare earths at high temperatures without 

 contact with any flame, and endeavours have already been made to 

 effect this by heating the oxides in specially constructed furnaces. 

 Under these conditions, only very minute differences could be detected 

 in the amount of light given by different oxides and mixtures, but it 

 appears doubtful whether the very high temperature of the Bunsen 

 flame was really attained. 



It has occurred to the writer, that very high incandescence could be 

 produced by enclosing mantles in a vacuum tube, and subjecting them 

 to bombardment by means of cathode rays, when the mantles would 

 not be in contact with anything except the cathode rays themselves, 

 and the comparatively small amount of residual gas that remains in 

 the tube at the requisite high degree of exhaustion. 



Since the date of Sir William Crookes's early researches, it has been 

 known that a very high temperature could be produced in a body 

 placed at the focus of the convergent rays from a concave cathode. In 

 this manner Crookes melted platinum and glass, and brought carbon 

 wool to bright incandescence. The writer has made many experi- 

 ments on this subject, using instead of the interrupted continuous 

 currents employed by previous investigators, alternating electric- 

 currents, which appear to. have many advantages for this purpose. 

 At the Eoyal Institution, in February, 1898, the writer showed that 

 very brilliant incandescence could be obtained for a short time in a 

 small block of lime, placed in a suitably exhausted tube midway 

 between two concave electrodes, connected to an alternating electric- 

 supply at about 6000 volts pressure, and in June, 1898, at the Boyal 



K 2 



