CHEMISTRY 89 



a particle is electrified it is very easily identified, whereas an 

 uncharged molecule is most elusive ; and it is only when these 

 are present in immense numbers that we are able to detect 

 them. A very simple calculation will illustrate the difference 

 in our power of detecting electrified and unelectrified mole- 

 cules. The smallest quantity of unelectrified matter ever de- 

 tected is probably that of neon, one of the inert gases of the 

 atmosphere. Professor Strutt has shown that the amount 

 of neon in -^ of a cubic centimetre of the air at ordinary 

 pressures can be detected by the spectroscope; Sir William 

 Ramsay estimates that the neon in the air only amounts to 

 one part of neon in 100,000 parts of air, so that the neon in 

 -~ of a cubic centimetre of air would only occupy at atmos- 

 pheric pressure a volume of half a millionth of a cubic centi- 

 metre. When stated in this form the quantity seems exceed- 

 ingly small, but in this small volume there are about ten mil- 

 lion million molecules. Now the population of the earth is 

 estimated at about fifteen hundred millions, so that the small- 

 est number of molecules of neon we can identify is about 

 7,000 times the population of the earth. In other words, if 

 we had no better test for the existence of man than we have 

 for that of an unelectrified molecule we should come to the 

 conclusion that the earth is uninhabited. Contrast this with 

 our power of detecting electrified molecules. We can by the 

 electrical method, better by the cloud method of C. T. R. 

 Wilson, detect the presence of three or four charged particles 

 in a cubic centimetre. Rutherford has shown that we can 

 detect the presence of a single a particle. Now the a particle 

 is a charged atom of helium ; if this atom had been uncharged 

 we should have required more than a million million of them, 

 instead of one, before we should have been able to detect 

 them." 



The "corpuscles" which are all of the same kind and can 

 be obtained from all substances are units of negative electric- 



