332 POPULAR SCIENCE MONTHLY 



To bring the essential features of this remarkable experiment 

 before you, I must begin some way off by reminding you of several 

 things you already know. For instance, the quantity of water vapor 

 which a given volume of air at ordinary pressures can hold without 

 depositing it as a mist or rain increases with the temperature. If air 

 enclosed in a vessel is allowed to expand suddenly its temperature falls. 

 If the air were initially saturated with water vapor, after the expansion 

 some of the vapor will go into mist or rain, provided any nuclei are 

 present upon which the excess vapor can condense. In the ordinary 

 fog or shower the dust particles always present in the open air act as 

 nuclei for the formation of drops. Small free charges of electricity 

 or ions serve the same purpose and the negative ions are more effective 

 condensers than the positive, hence they come down first. 



In a complicated vessel, which need not be described, Professor 

 Thomson admitted dust-free air saturated with water vapor. This 

 mixture was allowed to expand several times to make sure of freeing 

 it from accidental dust or ions which might be present. The former 

 pressure was then restored and the gas ionized by admitting X-rays 

 through the thin aluminum lid of the gas chamber. The next expan- 

 sion, chosen sufficient in amount to cause condensation on the negative 

 but not on the positive ions, caused a copious cloud of mist which 

 gradually settled by its own weight to the bottom of the vessel. The 

 top of the cloud as it fell was sharply defined, and its rate of descent 

 could be measured. 



Sir George Stokes many years before had calculated the rate of fall 

 of small spherical bodies through air, and one needed to know only the 

 density of a small sphere and its rate of fall to compute its size. The 

 approximate volume of the individual drops could thus be found. The 

 quantity of water in the whole shower could also be easily determined, 

 hence the number of drops, equal to the number of negative ions upon 

 which they might form, could be calculated. 



In another way Professor Thomson could measure the total quan- 

 tity of free negative electricity present in the chamber when the fog 

 was precipitated. He had thus the number of negative ions and the 

 sum of their charges, and therefore the charge each carried. 



The charge Professor Thomson found as the result of his brilliant 

 experiment was the atom of electricity over again. After this it was 

 impossible to escape the conclusion that the bodies flying in the cathode 

 stream were masses no greater than the one one-thousandth part of the 

 hydrogen atom. Thus matter, or electricity, or something exists, which 

 measured by inertia is a thousand times smaller than the lightest known 

 atom of matter. Furthermore, the kind of gas in which the cathode 

 discharge took place had no effect upon either the charge or the mass 

 of the particles, which bear no observable earmarks to reveal the kind 



