THE SIZE OF ATOMS. 151 



exceeding smallness of atoms. Atoms are not 

 so exceedingly small after all. The four lines 

 of argument I have referred to make it perfectly 

 certain that the molecules which constitute the air 

 we breathe are not very much smaller, if smaller 

 at all, than i/io,ooo,oooth of a centimetre in 

 diameter. I was told by a friend just five 

 minutes ago that if I gave you results in cen- 

 timetres you would not understand me. I do 

 not admit this calumny on the Royal Institution 

 of Great Britain ; no doubt many of you as 

 Englishmen are more familiar with the un- 

 happy British inch ; but you all surely under- 

 stand the centimetre ; at all events it was taught 

 till a few years ago in the primary national 

 schools. Look at that diagram (Fig. 33), as I 

 want you all to understand an inch, a centimetre, 

 a millimetre, the i/ioth of a millimetre, the 

 i/iooth of a millimetre, the i/ioooth of a 

 millimetre, and the i/i,ooo,oooth of a millimetre. 

 The diagram on the wall represents the metre ; 

 below that the yard ; next the decimetre, and a 

 circle of a decimetre diameter, the centimetre, and 



