in the EartJfs Surface Materials. 761 



A slow current of air, entering the flask at E, carries the 

 evolved emanation of thorium to the electroscope. On its 

 way the air passes through the drying-tube, t, containing 

 phosphoric anhydride ; the current being steadily maintained 

 by a water-suction pump working under a constant head. 



The velocity of flow is not such as to interfere with the 

 observation of the gold leaf. A thin copper tube, with finely 

 perforated walls, diffuses the entering air current; and the tube 

 by which it is withdrawn reaches nearly to the bottom of the 

 electroscope. These latter details are not shown in the figure. 



In order to reproduce on each occasion the same velocity 

 of nir-flow, a stopcock, 0, and a manometer tube, m (which 

 dips into a vessel of oil), are placed between the condenser 

 and the drying-tube. The stopcock serves to introduce an 

 obstruction to the current of air, so that a certain rarefaction 

 (indicated by the height to which the column of oil is drawn 

 in m) must be established to the right of the obstruction in 

 order that the flow may take place. By this arrangement 

 variations in the resistance of the drying-tube will not affect 

 the indications of the manometer. In a more recent form of 

 the apparatus the U drying-tube is replaced by a shorter 

 straight tube, and a second very short tube, containing granu- 

 lated calcium chloride, is placed between the stopcock and 

 the condenser. This latter tube serves to protect the stop- 

 cock from moisture. 



The period of the emanation of thorium being only 54 

 seconds, it is evidently of importance to secure its rapid 

 evolution from the solution and its immediate transfer to the 

 electroscope. The first condition seems secured by the pro- 

 cess of ebullition. The second is best attained bv an air-flow 



