xv THE MOLECULAR THEORY 341 



For the purposes of the argument it is not necessary to 

 know the actual number of molecules in the litre of gas, 

 but recent experiments have shown that the number n has 

 the value 3 x io 2 ' 2 , the probable error in this number being 

 only about io per cent. The " Avogadro constant," A", 

 which represents the number of atoms in the gram-atom or 

 of molecules in the gram-molecule, e.g. oxygen atoms in 

 1 6 grams of oxygen, or oxygen molecules in 32 grams of 

 oxygen, has been found to be 67 x io' 22 , ten different 

 methods of measurement having given values lying between 

 62 and 72 x io 22 . 



Complexity of gaseous molecules determined by means of 

 Avogadro's hypothesis, The fundamental weakness of 

 Dalton's Atomic Theory arose from the fact that he had no 

 way of finding out how many atoms were present in the 

 naolecule of a given compound. He always chose .the 

 simplest formulae that were possible ; but this procedure 

 was quite arbitrary, and it was not possible to say whether 

 the conclusions arrived at were correct or not. Avogadro's 

 hypothesis had the merit of supplying a method by which 

 the relative weights of the atoms, and their relative numbers 

 in any given compound, could be determined from the 

 properties of the substances themselves, instead of from the 

 arbitrary whim of the investigator. The following examples 

 were selected as illustrations of the new method : 



Water. Dalton had supposed that water was formed by 

 the union of one atom of hydrogen with one atom of 

 oxygen. Avogadro recognising that :. 



2 volumes of hydrogen + i volume of oxygen yield 2 

 volumes of steam 



and that, therefore : 



2 molecules of hydrogen + i molecule of oxygen yield 2 

 molecules of steam, 



