on Ionization Currents. 127 



gas left in the reservoir A may be extracted by lowering the 

 mercury in D and repeating the process. It will be noticed 

 that after the mercury has pushed the gases beyond F, the 

 emanation does not come in contact with the grease of any 

 stopcock. This is important, for the emanation decomposes 

 the grease-producing gases. The heating of the copper and cop- 

 per oxide should be continued until the pressure indicated by 

 the columns of mercury in the tubes C ceases to diminish, after 

 which the copper and copper oxide are allowed to cool, and 

 the tube G is surrounded by liquid air. When the emanation 

 has been sufficiently condensed in G, the communication with 

 the mercury pump through the tubes C should be opened sev- 

 eral times, for a few seconds only each time, until a high vacuum 

 has been obtained. If this communication is left open a long 

 time, part of the emanation evaporates into the pump and is 

 lost. Finally, on removing the liquid air the mercury may be 

 allowed to now into the tubes through F and afterwards 

 through C, pushing the emanation up into the little bulb H. 

 The tube G should be large enough for the mercury to drop 

 into without imprisoning emanation. 



Using this method, it is not difficult to condense the quantity 

 of emanation in equilibrium with 0*2 gram of radium chloride 

 into a volume of 0*2 — 0*3 cubic millimeters. This volume is 

 considerably larger than that of the emanation itself, and 

 therefore the latter is not chemically pure. The process of 

 condensation, however, need not last longer than about two 

 hours, which is advantageous for certain purposes. To obtain 

 the emanation chemically pure, the gases should be left in con- 

 tact with the chemicals for a long time, and some such process 

 of fractional distillation as those employed by Rutherford* 

 and Debiernef should be used. 



The effects produced by the magnetic field on the ioniza- 

 tion currents may be stated briefly as follows : In the first place 

 there is an enormous difference between the positive and nega- 

 tive currents. The magnetic force diminishes the value of 

 the negative current very appreciably even at pressures of 

 several centimeters ; and at low pressures (less than 0'06 mm ) a 

 field of a few hundred gauss reduces it to a small fraction of one 

 per cent of its value. This is true of hydrogen as well as air. 



On the other hand, I have not observed any effect of the 

 magnetic field on the positive current in air with pressures 

 above O03 nm , except that there is always an initial drop of 

 about 5 per cent for small magnetic fields, due undoubtedly to 

 the suppression of (3- and perhaps secondary rays. 



If the pressure is of the order of several thousandths of a 



* Phil. Mag. 1908. 



f See Madame Curie, Traite de Radioactivite, I, pp. 312-321. 



