Air and IJt/droaen hy heat'imj a Platinum Wire. 315 



hydrogen, either with exininsions below or above the vahie 

 1*25 necessary to catch negative ions. The })otentials tried 

 in this experiment were 2, 10, 40, 80, 120 volts positive and 

 negative. 



From this result, therefore, the nuclei produced by the 

 heating of platinum wire in air and hydrogen are uncharged. 



\Vhat these nuclei actually are it is impossible to say witli 

 absolute confidence. It is highly improbable that they are 

 due to dirt on the wire. On one occasion a wire was kept 

 red-hot for thirteen hours with a stream of filtered air flowing 

 along it, and yet afterwards fogs w^ere got by heating it 

 to a temperature below 200° Q. 



There is an interesting analogy between the results described 

 in this paper and those got by Dr. Walter Stewart * on the 

 disintegration of platinum wires at high temperatures. 



He found that a platinum wire at a clear red heat in air 

 lost O'8-l per cent, of its weight in two hours ; at a white 

 heat it lost 3'89 per cent. 



In hydrogeu he found that the wire does not disintegrate, 

 even at a clear white heat. 



The fact that Aitken_, by heating a platinum wire in air ta 

 a red heat, got enough nuclei to give many dense fogs, 

 without the wire appreciably losing weight, is not surprising 

 when we consider the excessively small dimensions of the 

 nuclei under consideration. For from C. T. R. Wilson^s f 

 calculation the radius in cms. of waterdrops equivalent in 

 their action to the nuclei which are caught with expansions 

 of 1*25 is about 8*6 x 10~^, so that the nuclei are not much 

 larger than molecules. 



In view of the extreme delicacy of the expansion method 

 of detecting nuclei, the production of nuclei by the heating 

 of platinum wires may be looked upon as a case of dis- 

 integration of the wire. If this view of the results be taken, 

 it is interesting to notice that a platinum wire disintegrates 

 in air at a temperature below 300° C, and, contrary to 

 Dr. Stewart's result, it does disintegrate in hydrogen at 

 temperatures of the order of 1000° C. 



In conclusion, I wish to acknowledge my indebtedness and 

 sincere thanks to Prof. Thomson for his kind encouragement 

 and valuable suggestions while these experiments were in 

 progress. 



Cavendisli Laboratorv, 

 April 24th, 1903. ' 



* Phil. Majr. vol. xlviii. p. 481 (1899). 



t Phil. Trans. A. vol. clxxxix. p. 300 (1897). 



