MOLECULAR MAGNITUDES 277 



has been legally defined as the steady current which in 

 1 second will transfer 1 coulomb of electricity. The 

 coulomb, which is defined as one tenth of the quantity 

 transferred in 1 second by one absolute e.m. unit of 

 current, is legally defined as the quantity of electricity 

 which is transferred in electrolysis between two silver 

 plates in a slightly acid solution of AgNOs when 

 0.001118 gram of silver is deposited on the cathode. 

 Since the atomic weight of Ag is commonly accepted 

 to be 107.88, it follows that the deposition of 1 mole 

 of silver represents the transfer of 107.88/0.001118 or 

 96500 coulombs, or 9650 absolute e.m. units. 



The value of the charge carried by the electron is, 

 however, usually expressed in electrostatic units. 

 Represent the value of the electron in these e.s. units 

 as e. The e.m. unit of quantity is 3X10 10 times the 

 e.s. unit, as may be determined experimentally by 

 measuring the same charge of electricity first in one unit 

 and then in the other. 1 By 1 mole of silver there is, 

 then, transferred 9650(3 X10 10 ) or 28,950 X10 10 abso- 

 lute electrostatic units of electricity. If N represents 

 the number of silver atoms (strictly ions) which are in 

 the mole, then in e.s. units this amount of electricity 

 is Ne. Millikan's value for e is 4.774 X10~ 10 , and hence 

 his value for N is 60.65 X10 22 . 



The mass of a molecule of any substance is to be 

 found as the quotient of its molecular weight in grams 

 and N. Thus for hydrogen the mass is 2.016/60.65 X 10 22 



1 The ratio is the velocity of light. This experimental fact was 

 one piece of evidence substantiating Maxwell's theories, which were 

 referred to on p. 124 as anticipating Hertz's discovery of electro- 

 magnetic waves of frequencies much smaller than those of light. 



