68 Presidential Address 



the inert gases of the atmosphere. Professor 

 Strutt has shown that the amount of neon in 

 1/20 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 1/20 

 of a cubic centimetre of air would only occupy at 

 atmospheric pressure a volume of half a mil- 

 lionth of a cubic centimetre. When stated in 

 this form the quantity seems exceedingly small, 

 but in this small volume there are about ten 

 million million molecules. Now the population 

 of the earth is estimated at about fifteen hun- 

 dred millions, so that the smallest number of 

 molecules of neon we can identify is about 7000 

 times the population of the earth. In other 

 frords, if we had no better test for the existence 

 of a man than we have for that of an unelectri- 

 fied molecule we should come to the conclusion 

 that the earth is uninhabited. 



The parable is a striking one, for on 

 these lines it might legitimately be con- 

 tended that we have no right to say posi- 

 tively that even space is uninhabited. 

 All we can safely say is that we have no 

 means of detecting the existence of non- 

 planetary immaterial dwellers, and that 



