46 DETERMINATION OF SPEED [CH. v. 



much residual gas in the tube it will likewise be 

 difficult to maintain a high potential difference ; 

 because that residual gas, under the influence of 

 the cathode rays, will conduct. Consequently the 

 best speeds are obtained at high vacuum ; and if 

 the density of the residual gas inside the tube is 

 constant, the speed will be constant. The nature of 

 the electrodes makes no difference, unless they give 

 off gas or otherwise make it difficult to maintain the 

 required potential difference. 



Although the speed of the particles in cathode 

 rays was thus found excessively great, their energy 

 was only moderate, and their aggregate mass was 

 therefore proved to be excessively minute ; their 

 aggregate electric charge, however, was considerable. 

 They were able to raise an electrical capacity of 

 15 microfarad several volts, sometimes as much 

 as 5 volts, in the course of a second ; and in the 

 same time they might be able to raise a calorimeter, 

 whose heat capacity was about 4 milligrammes of 

 water, by 2 C. Nevertheless their mass was so 

 small that it would have taken one hundred years 

 to collect a weighable amount : and then only 

 about one-thirtieth part of a milligramme. They 

 travelled with a velocity a hundred thousand times 

 greater than the speed of rifle bullets, and re- 

 presented the greatest velocity up to that time 

 observed in matter, if matter they were. And 

 the electrochemical equivalent, instead of coming 

 out in accordance with that observed in liquids, 

 came out some thousand times smaller ; that is to 

 say, the charge associated with each particle of the 

 cathode rays seemed a thousand times greater, in 

 proportion to the mass, than the charge associated 

 with an electrolytic ion, even of hydrogen. 



