206 Dike — Recent Observations in Atmospheric Electricity. 



heated to redness. Both methods were tested by Satterly at 

 the Cavendish Laboratory and careful series of observations 

 were made. Using the absorption method, the pulverized char- 

 coal was packed in porcelain tubes between asbestos plugs and 

 the air drawn through at a constant, measured velocity by 

 means of a filter pump, beiug freed from dust and dried by 

 means of calcium chloride before reaching the charcoal. The 

 air current was measured by a pressure gauge. After the air 

 had passed through for a known interval of time (the volume 

 per minute being known) the tube was placed in a furnace and 

 heated to redness. Air freed from emanation was passed 

 through the tube while hot, sweeping out the released emana- 

 tion, and was collected in aspirators over water, where it was 

 stored till ready for the test. The testing vessel was in the 

 meantime tested for air and insulation leaks, the air being 

 pumped out of the vessel and fresh air allowed to enter several 

 times. The air was finally exhausted from the testing vessel 

 and communication made w T ith the aspirator containing the air 

 loaded with emanation, which then passed into the testing 

 vessel. The leakage current was measured for 20 minutes, 

 not waiting for the 3-hour maximum. 



In order to refer these results to radium as a standard, air 

 was bubbled through a solution containing a known amount 

 of radium, thus removing the emanation as it was given off. 

 This went on simultaneously with the collection of emanation 

 from the air, a similar charcoal tube being used for absorbing 

 the radium emanation. This was treated in the same way as 

 the other and a similar leakage test made. This second test 

 gave of course the radium emanation plus the emanation from 

 the air, but since the observations were simultaneous the air 

 effect could be allowed for. The comparison gives the ratio 

 oi; the emanation in a known volume of air to the emanation 

 generated in the solution in a known time. The results must 

 be corrected for decay of the emanation if not measured for 

 some time after it was absorbed, as was sometimes the case, and 

 for the gradual accumulation of emanation in the charcoal. 

 The amount of emanation generated in a given time by the 

 solution must be calculated. The resulting mean from eight 

 measurements gives as the amount of radium required per 

 cubic meter of air to keep up the quantity of emanation to the 

 observed amount 88X10 -12 grams with a range from 50 XlO -12 

 to 160 XlO -12 . The comparison of day with night values seem 

 to indicate- more emanation during the day than at night. 



For the liquid air condenser method, a special condenser was 

 devised with a large cooling surface and small resistance to the 

 air current, consisting of a brass tube packed as full as possible 

 with thin straight brass wires. This was immersed in liquid 



