OBSERVATIONS ON RADIO-ACTIVITY. 385 



As Madame Curie found that the activity of uranium 

 oxide U 2 Oo was constant for a period of five years, we 

 compared the current passing when the mineral was tested 

 with that from uranium oxide, the same standard specimen 

 being used for each comparison. This was done by observ- 

 ing the time with a stop clock, which the leaf took to fall 

 through the same distance, first with the mineral then with 

 the U 2 0.5. In all cases the minerals were finely ground, 

 and the lead dish completely filled and levelled off. 



In examining the emanation from several specially 

 selected minerals, about 40 grams of the finely powdered 

 substance was heated in a porcelain tube in an organic com- 

 bustion furnace to the highest attainable temperature. 

 The tube was pumped out whilst thus heated, allowed to 

 fill and air drawn over from the other end, and again pumped 

 out. The gas from the Sprengel pump was collected over 

 mercury, and transferred to a partially evacuated metal 

 vessel, of about 800 cubic centimetres capacity, with a 

 central insulated wire. The gas in the vessel was brought 

 to atmospheric pressure by the addition of air. The walls 

 of the vessel were kept at a positive potential of 300 volts, 

 and the current through the gas to the wire, attached to 

 the gold leaf of the Wilson electroscope, charged it posi- 

 tively with resulting fall of the leaf. 



Measurements were taken over a period of four days and 

 the results plotted. Decay of the radio-activity of the 

 emanation to half its initial value in four days indicates, as 

 shown by Rutherford and Soddy, the presence of radium. 

 Our instrument was not suited for measuring the decay of 

 the thorium emanation. In this work Rutherford's " Radio- 

 activity" was found most useful, especially the chapter on 

 methods of measurement. 



Y— Oct. 5, 1904. 



