POPULAR SCIENCE 477 



exact value for the Avagadro Constant. We may take this 

 value as 6"ix io 23 . If we pause to consider the enormous 

 magnitude of this quantity, a quantity which is known at the 

 same time with a considerable degree of precision, we cannot 

 but be struck by the great advance which has been made 

 in our knowledge of the molecular world as a result of 

 such researches. The above number means that in any gas, 

 which obeys the gas law, the number of molecules present in 

 one cubic centimetre at normal temperature and pressure is 

 very nearly 2*8 x io 19 . The mass of a single molecule — of 

 hydrogen, say — is also easily calculable from the data given 

 and comes out to be 3*2 x io~ 2 * gram. That is, one gram of 

 hydrogen contains 300,000,000,000,000,000,000,000 molecules. 

 To give a somewhat more tangible, though less exact, idea of 

 what such numbers represent, we cannot do better than recall 

 the classic illustration of Lord Kelvin, viz. if we imagine a 

 small drop of water magnified to the size of the earth, the 

 molecules would then be visible, their size being somewhere 

 between that of a cricket-ball and that of small shot. 



The Gaseous and Liquid States 



Whilst we are on the subject of molecular magnitudes, it 

 will be convenient to recall a few other related quantities which 

 have become known to us as a result of investigations of the 

 gaseous state. The first of these is the diameter of a mole- 

 cule. Knowing the number of molecules per cubic centimetre, 

 it is possible to calculate the diameter of each by making use 

 of viscosity data. The following table contains a few of the 

 values obtained in this way by Sutherland (4) : 



It will be seen that the dimensions of a molecule are extra 

 ordinarily minute, so minute, in fact, that if we imagine a 

 number of hydrogen molecules placed end to end, it would 

 require fifty million of them to form a row one centimetre in 



