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Professor Sir J. J. Thomson 



[April 14, 



There is thus considerable difference between the quality of the 

 rajs from the celluloid target and those from the lead and aluminium 

 ones. There is little difference, however, between those from lead and 

 aluminium for the higher voltages, nor did I find any very marked 

 difference in the quality of the Rontgen rays produced when the 

 same number of cathode rays of the same speed struck lead and 

 aluminium targets respectively. 



AVith rays of this very soft type it is only a very thin layer close 

 to the surface of the target which is operative, and so the cleanliness 

 of the surface is a matter of great importance. If different sub- 

 stances give out characteristic radiation of the kind produced by these 

 slowly moving cathodes, we should expect that the absorption by 

 these substances would show anomalies when the rays passing through 

 them approached in type to their characteristic radiation, and that the 

 ratio of the absorption of different substances wouLd show consider- 

 able variation. This was a very marked feature in these experiments. 

 If, for example, we compare the absorption of the rays by gold and 

 aluminium, the absorption by aluminium for very soft rays — say for 

 those corresponding to voltages of 400 or 500 — is in comparison with 

 gold many times greater than for rays corresponding to more than 

 1500, so much so that a piece of aluminium foil which does not 

 absorb nearly so many of these harder rays as a piece of gold-leaf 

 may absorb three or four times as many of the softer rays. The 

 following examples show how marked is this effect ; the plates of the 

 different metals were not of the same thickness, so that the results 

 do not give the absolute values of the absorption nut only the varia- 

 tion of the absorption with the voltage ; the target was silver. 



Thus with the highest voltage the absorption of the gold is twice 

 that of the aluminium, while with the lowest it is only about one-sixth ; 

 this points to the existence of a soft aluminium characteristic radia- 

 tion, and also probably to one for copper. 



The next question I shall consider is the variation in the energy 

 of the Rontgen radiation with that of the cathode rays which produce 

 them. We must remember that in the method we are considering at 

 present the Rontgen rays have to pass through a thin film of 

 celluloid before their energy is measured, so that any energy absorbed 



