156 



Scientific Proceedings, TRoijal Dublin Socieiij. 



3 ems. in diameter. Tlie other is open and rests upon the ground, the edge 

 being pressed in, as shown, when an experiment is progressing. Within this 

 collector there is a disk of tin plate resting on feet which raise it about 3 cms. 

 off tlie soil. It is about 1 centimetre less in diameter than the collector. 

 The disk is centrally perforated, and carries a vertical tube which emerges 

 through the opening on top of the collector. The air-suction is applied to 

 this pipe. This evidently gives rise to air-currents within the collector which 

 pursue the course shown by the arrows. Of course it is impossible to secure 

 uniformity in the currents ; but, on the whole, there must be a radial and 

 inward draught beneath the disk. As the entry of air is perfectly free and 

 the rate of air-suction slow, there is no sensible reduction of pressure beneath 

 the collector. This is shown by the fact that variations in the air-suction rate 

 do not seem to sensibly affect the results, some of the highest being obtained 

 with air-currents at the rate of 7'5 litres per hour, and some of the lowest 

 with rates of about 25 litres per hour. The experiment was also tried of 

 placing a thin cardboard disk centrally on the ground just beneath the 

 uptake tube. No difference in the results was noticed. 



In order to collect the emanation withdrawn during an hour's application 

 of the air-draught, the air-current drawn from the collector is led over cocoa- 

 nut charcoal contained in a quartz tube. The emanation is thus, for the 

 greater part, absorbed. The quartz tube (a porcelain tube was also on some 

 occasions used) is about 1"5 cms. internal diam., and 50 cms, in length. The 

 central part is filled with about 50 grams of the charcoal, not too finely 

 pulverized. Loose asbestos fills the remainder of the tube. After the draught 

 has run for an hour this tube is attached to a strong indiarubber bag and 

 heated in a gas-furnace for about forty minutes. The contents of the bag are 

 then admitted into an exhausted electroscope. The emanation is determined 

 by observation of the electroscope as already described. 



