126 Prof. Thomson & Mr M c Glelland, On the Leakage of [Mar. 9, 



But the difficulty of convincing the public or even the scientific 

 world of the authenticity and genuineness of any of the specimens 

 was greatly increased by the enormous mass of false or incon- 

 clusive evidence that was then adduced in favour of the discovery. 

 Those therefore who believe most firmly that traces of man earlier 

 than those now known as paleolithic will be found, should be the 

 most careful in their examination of the evidence adduced while 

 the enquiry is in progress. 



(5) On the Leakage of Electricity through Dielectrics tra- 

 versed by Rontgen Rays. By Prof. J. J. Thomson, M.A., F.R.S., 

 and J. A. McClelland, Fellow of the Royal University of 

 Ireland, Trinity College, Cambridge. 



Experiments proving that dielectrics when traversed by 

 Rontgen rays conduct electricity were described by one of us 

 in papers read before the Philosophical Society on Jan. 28, 1896, 

 and before the Royal Society on Feb. 13, 1896. The following 

 paper contains an account of further experiments made to in- 

 vestigate the laws governing the passage of electricity through 

 dielectrics exposed to Rontgen's rays. 



Method of making the Experiments. The induction coil and 

 the bulb for producing the rays were inclosed in a large iron tank, 

 fitted with a metallic lid. A small hole was bored in the lid and 

 the bulb placed underneath this hole. The radiation from the 

 bulb passed through the hole and fell upon the dielectric whose 

 conductivity was to be investigated. In the experiments with 

 gases two different methods were employed; in the first of these, 

 which we shall call method (A), the lower end of a piece of glass 

 tubing about 4 cm. in diameter was closed by a piece of thin 

 sheet aluminium, a paraffin stopper was placed in the upper end 

 of the tube, a stout metallic rod passed through the paraffin and 

 supported a metallic disc with its plane parallel to the plate of 

 aluminium. The rod and disc were connected with one pair of 

 quadrants of a quadrant electrometer. The plate of aluminium 

 rested on the iron tank, which was connected with the earth. To 

 determine the rate of leak through the dielectric the two pairs of 

 quadrants of the electrometer were connected together and with 

 one terminal of a battery of small storage cells, the other terminal 

 of which was connected with the earth. The quadrants of the 

 electrometer were then disconnected from each other and from the 

 battery. Before the Rontgen rays were turned on the insulation 

 was so good that there was no movement of the spot of light 



