22 FORESHADOWING OF THE ATOM [CH. HL 



the electrochemical equivalent of hydrogen, being 

 Y^-gth of this quantity, is 



4*025 grammes 4*025 



108 ampere-hours = 108 x 360 C ' g ' S * 



= '0001035 c.g.s. = Q gg nn grammes per coulomb. 



Hence the ratio of an atom of electricity to an atom 

 of hydrogen is 9,660 M~^ c.g.s. units, or approximately 



centimetres\ 



{ // 

 V 



* grammes/ 



the unknown constant M necessarily making its 

 appearance, because we are comparing quantities of 

 different nature, or at any rate quantities measured 

 in different ways, viz., 'electricity' and 'matter' (see 

 Appendix D). 



The numerical part of this quantity is known 

 with comparative exactitude,* that is to say up to 

 the limits of error of experiment : to proceed 

 further, we must make an estimate of the mass of 

 an atom. That can be done, and has been done, in 

 many ways, and we have been taught both by Dr. 

 Johnston e Stoney and by Loschmidt, originally even 

 by Dr. Thos. Young, but with greatest force and range 

 by Lord Kelvin, that the mass of an atom of water is 

 approximately 10~ 24 of a gramme; wherefore an atom 

 of hydrogen will be approximately 10 ~ 25 gramme; 

 whence the unit of electric charge is 10~ 21 c.g.s. 

 magnetic unit, or 10~ 10 of an electrostatic unit or 

 10- 20 of a coulomb. 



*The decimal places are correctly printed above ; though the fact 

 that 1 coulomb, or 1 ampere-second, is one-tenth of a c.g.s. unit owing 

 to the ohm and volt having been inadvertently defined, one as 10 9 , and 

 the other as 10 8 c.g.s., instead of both the same always stands ready to 

 introduce confusion and error. 



