A HISTORY OF SCIENCE 



make it a conductor of electricity. I cannot go into 

 details here as to the studies that have been addressed 

 to the answer of this question, but I will briefly epit- 

 omize what, for our present purpose, are the important 

 results. First and foremost of these is the fact that a 

 gas thus rendered conductive contains particles that 

 can be filtered out of it by passing the gas through 

 wool or through water. These particles are the actual 

 agents of conduction of electricity, since the gas when 

 filtered ceases to be conductive. But there is another 

 way in which the particles may be removed namely, 

 by action of electricity itself. If the gas be caused to 

 pass between two metal plates, one of them insulated 

 and attached to an electrometer, a charge of positive 

 electricity at high potential sent through the other 

 plate will drive part of the particles against the in- 

 sulated plate. This proves that the particles in ques- 

 tion are positively electrified. The amount of the 

 charge which they carry may be measured by the 

 electrometer. 



The aggregate amount of the electrical charge car- 

 ried by these minute particles in the gas being known, 

 it is obvious that could we know the number of particles 

 involved the simplest calculation would determine the 

 charge of each particle. Professor Thompson devised 

 a singularly ingenious method of determining this num- 

 ber. The method was based on the fact discovered by 

 C. T. R. Wilson that charged particles acted as nuclei 

 round which small drops of water condense much as 

 dust particles serve the same purpose. " In dust-free 

 air," says Professor Thompson, "asAitken showed, it 

 is very difficult to get a fog when damp air is cooled, 



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