Sir Ernest Rutherford 229 



Supposing that in an ordinary electric light bulb a hole 

 could be punched small enough to let in a million atoms of 

 oxygen a minute, how long do you think it would take to 

 fill the bulb ? The answer is one hundred million years ! 



Lord Kelvin has given us another calculation which 

 makes us realize the minuteness of the atom. Write 

 down twenty and follow it by eighteen noughts. This 

 gives the number of molecules (not atoms) which occupy 

 one cubic centimetre at freezing-point. Since a cubic 

 centimetre represents about one-third the contents of an 

 ordinary thimble, it follows that a thimble will hold 

 60,000,000,000,000,000,000 molecules, each of which, in 

 the case of hydrogen, consists of two atoms. The total 

 weight of this almost incredible number of molecules is 

 about the one-seven-hundredth part of a grain. 



In order to begin his investigations concerning the atom 

 Faraday had gold beaten into leaf which was only one- 

 millionth of an inch thick. Yet this film, so fine as to be 

 quite transparent, was estimated to contain between ten 

 and fifty layers of molecules, and a molecule, remember, is 

 built up of several atoms. It was necessary to get some- 

 thing thinner even than the fragile gold-leaf, and this was 

 done by putting a drop of oil on to a large basin of water. 

 The film thus formed was about a millionth of an inch 

 thick. At first this film showed the marvellous variety of 

 prismatic colours which you may see upon a wet road 

 where oil from a car has been spilled, but as the film grew 

 thinner a black spot appeared, the blackness indicating 

 that the film was then too thin to reflect any light. This 

 black portion of the film provided material from which 

 the maximum size of atoms could be calculated. 



Even so, no one has ever seen an atom, and probably 

 no human eye ever will, and that being so it seems incredi- 

 ble that we know so much about these complicated little 

 bodies. For our knowledge we are indebted to the long 

 and careful experiments of men, such as Sir Ernest 



