ATOMS AND MOLECULES 45 



a minute object as a blood corpuscle contains, 

 according to Wismann, about 3,625,000,000 large 

 molecules ; and it is calculated that no living germ 

 can contain fewer than 100,000,000 large molecules. 

 So light are molecules, too, that it requires 

 1,000000,000000,000000 molecules to affect a 

 delicate balance. 



Figures such as these convey very little to the 

 mind, and efforts are often made to express the 

 multitude and minimissitude of molecules in some 

 comprehensible way. Thus Sir Oliver Lodge 

 explains that " a portion of substance consisting of 

 a billion (a million million) atoms is only barely 

 visible with the highest power of a microscope ; 

 and a speck or granule, in order to be visible to 

 the naked eye, like a grain of " lycopodium-dust, 

 must be a million times bigger still." Kelvin, 

 again, tries to give an idea of the smallness of the 

 atom by an estimate that if a drop of water were 

 magnified up to the size of the earth, the atoms 

 would be larger than pellets of shot, and smaller 

 than cricket-balls. Others have pointed out that 

 we know dimensions small enough to be compar- 

 able to molecules — that the thinnest part of a soap- 

 bubble is only about twenty or thirty molecules 

 thick, and that gold-leaf and films of oil may be 

 prepared only about seven or even fewer molecules 

 thick. (It might also be safely asserted that a 



