produced by the Action of Thorium Compounds. 189 



in 15*5 sees, to 100 divisions in 20 sees. After 5 minutes' boiling 

 in caustic soda, the rate of discharge fell to 100 in 27 sees. After 

 10 minutes' exposure to strong hot nitric acid, the rate of dis- 

 charge was cut down to one-half its previous value. Dilute 

 sulphuric acid reduced the rate of discharge to one-half in 

 10 sees, and one-quarter in 60 sees. Both hydrochloric and 

 sulphuric acids are more powerful in destroying the radio- 

 active power than the other solutions examined. In the case 

 of water and caustic soda, the small diminution of intensity 

 appears to be due as much to the mechanical action of the 

 bubbling as to the chemical action on the surface. 



The question now arises, whether the loss of radioactivity 

 of the active plate by immersion in solutions is due to the 

 destruction of the radio-active power of the particles or their 

 removal from the plate to the solution. A fine platinum wire, 

 very strongly active, was placed in a few drops of dilute 

 sulphuric acid for several minutes. The wire lost a large 

 proportion of its radioactivity. The dilute acid was then 

 evaporated down to dryness in a sand-bath, and on exami- 

 nation it was found that the residue on the glass surface was 

 strongly active. We may conclude from this experiment 

 that the radioactivity of the particles is not destroyed, but 

 that they pass into solution, and that on evaporating the 

 solvent the substance still remains. 



Some experiments were tried to see whether a plate preserved 

 its radio-aotive power when a layer of copper was electrolyti- 

 cally deposited upon it. A radio-active platinum wire was 

 made a cathode in a copper-sulphate solution, and a current 

 of about half an ampere passed through for 1 minute. The 

 radioactivity was diminished to about -7 of its value when tested 

 in the usual way. After washing the wire in water, it was 

 allowed to stand some time in air, and the rate of diminution 

 of the radioactivity observed. The intensity diminished 

 more rapidly at first than for an unacted-on wire; but after 

 10 hours the rate of diminution became normal. The more 

 rapid decrease at first is probably due to the dilute sulphuric 

 acid which remained in the pores of the copper deposit. 

 When the platinum wire was made the anode in a copper- 

 sulphate solution, the radioactivity rapidly diminished. The 

 action in this case was probably due to the production of 

 sulphuric acid at the surface of the anode by the passage of 

 the current which dissolved the radio-active material on the 

 platinum plate. 



