70 sir W. Crookes and Prof. J. Dewar. [May 28, 



1 Radium salt cooled by liquid air, fig. 1. Screen at ordinary 

 temperature. Scintillations quite as vigorous as with radium at the 

 ordinary temperature, the screen and radium being in vacua. 



2. Kadium at the ordinary temperature and screen cooled in liquid 

 air fig 2. As the screen cooled the scintillations became fainter and 

 at 'last could not be seen. On allowing the temperature to rise the 

 scintillations recommenced. 



3 A screen with a speck of radium salt in front of it was sealed in a 

 tube. Water was put in the other end of the tube and the tube sealed 

 on the pump. A good exhaustion was kept up and the water boiled 

 away, the vapour being condensed in phosphoric anhydride. The 

 tube was sealed off when a few fine drops of water were still remaining 

 in the tube. The scintillations were well seen in this saturated 

 aqueous vapour. The lower end of the tube was dipped in liquid 

 air, which instantly condensed the aqueous vapour and left a very 

 good vacuum. On now examining the scintillations they were if 

 anything brighter and more vigorous than at first. When liquid 

 hydrogen cooling was used instead of liquid air the action was equally 

 marked, showing that the highest vacuum that can be obtained by the 

 action of cold does not diminish the scintillations. 



In the upper part of the tube, away from the radium and screen, 

 two platinum wires were sealed to show the state of the vacuum. 

 The spark passed easily at the ordinary temperature, showing a reddish 

 line of aqueous vapour. When the other end of the tube was in liquid 

 air the spark refused to pass. 



4. It was thought that perhaps the passage of the induction spark 

 might have liberated some occluded hydrogen, so another tube similar 

 to the above was made without the platinum wires. Here also immer- 

 sion in liquid air, if it had any effect, brightened the scintillations, 

 and on replacing the liquid and cooling by liquid hydrogen no change 

 was observable. 



In order to test the activity of radium in rendering air electrically 

 conductive, some radium bromide was sealed up in a glass tube and 

 heated to the highest temperature the glass would stand, during the 

 production of as high a vacuum as the mercurial pump would give. The 

 whole tube was then immersed in liquid hydrogen contained in a 

 vacuum vessel. On bringing the radium in such a vessel into a room 

 in which a charged electroscope was placed, it began to leak when the 

 tube of radium surrounded with liquid hydrogen was some 3 feet 

 away, and was very rapid in its action when a foot away from the 

 electrometer. On immersing the tube containing the liquid hydrogen 

 with submerged radium in another large vessel of liquid air and 

 bringing the combination near the electroscope the action was the 

 same. 



The luminosity of the radium salt in liquid hydrogen was much 



