hy Hot Platinum in Air at Low Pressures. ^6^ 



than at temperatures lower than this. The conditions at 

 each temperature are not exactly comparable, so that this 

 result cannot be reoarded as satisfactorily established. The 

 author hopes to make further experiments on this point in 

 the near future. 



The results do, however, clearly show not onlj' that the 

 leak at a given temperature is much smaller if the wire has 

 previously been heated to a higher temperature, but also 

 that the rate of decay is much less than before. For instance, 

 on first heating the wire to 751° C. it gave a leak of 360,000 

 divs. per half minute, which fell to 68,500 in 8 minutes ; 

 whereas after heating for about 20 minutes to 809° C. the 

 leak at 751° C. had become 5000 divs. per half minute, and 

 suffered no appreciable diminution on heating for 8 minutes 

 longer at the same temperature. 



It is evident from these experiments that to get rid of the 

 part of the leak which dies away rapidly, it is only necessary 

 to heat the wire for some time at a temperature somewhat 

 higher than that at which it is to be used subsequently. It 

 was found, however, that the steady leak thus obtained also 

 generally died away in time, only much more slowly ; so 

 that the distinction, at low^ temperatures and pressures at 

 any rate, is one of degree rather than kind. The complexity 

 of the phenomena generally, and especially of the rate of 

 decay of the leak, seem to indicate that there are several dis- 

 tinct substances present in the metal which are capable of 

 giving rise to positive ions. 



The fact that the positive ionization from hot wires be- 

 haves as if it were due to some substance evaporating from 

 the wire led the author to examine whether a negative 

 electrode which had been kept near a hot wire charged 

 positively became radioactive. The hot wire was maintained 

 at a temperature of about 737 degrees centigrade for one 

 hour, and was charged to 4- 200 volts, the cylindrical elec- 

 trode being earthed. The pressure of the gas remained 

 constant and =*05 mm. During the whole of this time 

 there was a steady current from the positive wire to the 

 cylinder ; so that it was thought that if the substance w^hich 

 disappeared from the hot w^ire were of the nature of an emana- 

 tion it would be collected on the negatively-charged cylinder. 



At the end of an hour the heating of the wire was dis- 

 continued and air was let into the apparatus. Experiments 

 were then made to see if there was any leak from the 

 cylinder to the cold wire. The results were purely negative ; 

 the current was at any rate less than 1/500 of an electro- 

 static unit. The experiments therefore lead to the conclusion 



