1907.] on High Vacua and Helium, 755 



Thorianite. 



The rare substance, thorianite, when heated gives off hehum. 

 Advantage of this was taken in the following experiment : A Crookes 

 radiometer, with the usual tube of charcoal attached, was provided 

 with an additional tube containing a small crystal of thorianite. On 

 immersing the charcoal bulb in liquid hydrogen and throwing on the 

 electric beam, no motion took place. The thorianite was, in these cir- 

 cumstances, heated by the flame of a Bunsen burner, and immediately 

 supplied enough helium to set the vanes in motion, and the instrument 

 remained active in spite of prolonged cooling of the charcoal «'ith 

 liquid hydrogen. 



Electric Discharge Radio^ieter. 



All the previous radiometers used were of the ordinary Crookes 

 pattern in which the mica vanes were blackened on one side. In the 

 experiment about to be described, a radiometer, with the usual char- 

 coal tube attached, was employed ; but it differed from the ordinary 

 radiometer in that the one side of each vane was covered with a thin 

 sheet of aluminium. The metallic frame bearing the vanes was con- 

 nected to one of the terminals of an induction coil, and the other 

 to a pole sealed through the glass of the radiometer. The bulb, 

 therefore, was a kind of discharge-tube containing a light mill. On 

 turning on the current, the vanes being made the negative pole, the 

 bulb lit up with a fine luminescence, and began to rotate rapidly. 

 But on immersing the charcoal bulb in liquid air, the vacuum was 

 greatly intensified, the glow became much diminished, and the rota- 

 tion of the vanes ceased altogether ; even the additional stimulus 

 of the beam from the electric arc was insufficient to produce any 

 motion. 



Experiment with Mercury Yapour to Measure Pressure 

 IN A Radiometer. 



During the experiments on high vacua it became abundantly 

 evident that the pressures reached were difficult to determine by 

 means of the McLeod gauge. Minute quantities of helium were to 

 be found everywhere — in the atmosphere, in the fine films of gases 

 condensed on the surfaces of glass vessels, on vanes and elsewhere. 

 It became, therefore, of importance to determine very small pressures 

 by other means, if possible. The radiometer experiments suggested 

 such a means, namely, by determining pressures below which the 

 radiometer would not spin. The pressures of mercury vapour have 

 been very accurately determined throughout a wide range of tempera- 

 ture. The following experiment shows how such measures can be 

 used to ascertain the limit of pressure referred to above. A Crookes 



