Condensation of the Radioactive Emanations. 563 



emanation is very remarkable considering the minuteness of 

 the actual amount of matter that must be involved. Arrange- 

 ments were made to investigate the phenomena in an accurate 

 quantitative manner. The method adopted was to condense 

 the emanations in a spiral copper tube and to employ the 

 latter as its own thermometer by determining its electrical 

 resistance. For this purpose a constant current was maintained 

 through the spiral, and the fall of potential between two fixed 

 points on the spiral was determined by means of a Weston milli- 

 voltmeter. This method proved very reliable and convenient. 

 A great number of preliminary experiments showed that to 

 obtain accurate results two requirements must be satisfied. 

 In the first place, since the temperature measured is the average 

 temperature of the whole spiral, the latter must be completely 

 immersed in a bath of liquid kept well stirred. In the second 

 place, in order to be sure that the spiral was not heated locally 

 by the entering gas-stream, it was necessary to subject the 

 latter to a preliminary cooling to the desired temperature. 

 This was accomplished by the apparatus represented in fig. 1 

 (PI. XIY.) . The spiral and connecting tubes were made out 

 of a continuous copper tube of length 310 cms., internal dia- 

 meter 2 mm., and thickness of wall -32 mm. This was first 

 wound into an inner spiral of 16 turns of mean diameter 

 1*80 cm., which was soldered together into a compact 

 cylinder. This effected the preliminary cooling of the gas- 

 stream. The tube was then wound back over the inner 

 spiral into an outer spiral of 14 turns of mean diameter 

 2' 90 cms. The turns of the outer spiral were separated from 

 each other and from the inner spiral by air-spaces. The 

 outer spiral constituted the thermometer, and the potential- 

 leads were soldered on to the top and bottom and inclosed in 

 glass tubes. The system was supported inside a glass cylinder 

 closed at the bottom, of height 41 cms. and diameter 3*5 cms., 

 by means of an air-tight rubber cork fitting the open end, 

 through w T hich the ends of the spiral and the leads passed. A 

 small stirrer at the bottom of the tube was driven by a central 

 rod passing through a suitable bearing supported by the inner 

 spiral, and through a glass tube in the centre of the cork. 

 This rod was operated by an electric motor supported above 

 the cork. The spiral was kept from actual contact with the 

 glass by a sheet of mica perforated into holes, and was given 

 sufficient rigidity by means of an ebonite ring fitting tightly 

 into the space between the upper part of the inside and 

 outside spiral. A current was sent (rig. 2, PL XIV.) through 

 the spiral by means of leads soldered above the cork from a 

 storage-battery, passing through a Weston ammeter and 



