﻿70 Mr. A. A. Campbell Swinton : Effect of Radium in 



60, another in the violet at wave-length 40, and others in the 

 ultra-violet. The first-order colour should be found at about 

 1'2 p. The radiometer gave a deflexion of several centi- 

 metres, when illuminated with the red band, immediately 

 dropping back to zero, as the prism was so turned as to bring 

 the infra-red region upon the slit. At about 1 fju a large 

 deflexion was obtained, which at 1*2 /x was " off the scale."" 

 The energy curve was then plotted from a large number of 

 readings, the slit having been narrowed in the meantime. 

 This curve is reproduced in fig. 2. The width and asymmetry 

 of the infra-red, or first-order band, are due to the fact that 

 the energy curve in this region of the spectrum is very steep 

 and asymmetrical. 



A crystal reflecting red, green, and violet bands at wave- 

 lengths 66, 50, and 40, should show two maxima in the infra- 

 red, one at 2 ^, the other at 1 //,. 



It seems probable that large sized flakes of chlorate of 

 potash may at times prove useful in research work when 

 it is desired to isolate a single spectral line, or cut out a 

 narrow region of a continuous spectrum. The advantage of 

 the flake over the spectroscope would lie in the large amount 

 of light available, which is often of great advantage, for 

 example in the study of the fluorescence of substances when 

 stimulated with monochromatic light obtained from metallic 

 arcs. The power of the crystals to reflect a comparatively 

 narrow band in the infra-red might sometimes be made use 

 of as well. In making crystallizations on a small scale, I 

 have seldom obtained satisfactory flakes much over 0*7 cm. 2 

 Larger ones could perhaps be made by working on a large 

 scale. 



XI. 21ie Effect of Radium in Facilitating the Visible 

 Electric l^iscliavge in Vacuo. By A. A. Campbell 

 Swinton *. 



AS has been shown by Edison, Fleming, and others, the 

 passage of the electric discharge in vacuo is much 

 facilitated by heating the cathode. 



More recently, as has been shown by Owen (Phil. Mag. 

 viii. 1904) and by Wehnelt (Phil. Mag. x . 1905), the 

 passage of the discharge is still further facilitated by coating 

 the heated cathode with oxides of the alkaline metals ; the 

 effect in this case being so great that large and highly 

 luminous discharges amounting to several amperes can, 



* Comuiuuicated by the Physical Society : read June 22, 190G. 



