Physical Units of Nature. 385 



And, finally, Nature presents us, in the phenomenon of 

 electrolysis, with a single definite quantity of electricity which 

 is independent of the particular bodies acted on. To make 

 this clear I shall express " Faraday's Law" in the following 

 terms, which, as I shall show, will give it precision, viz.: — For 

 each chemical bond which is ruptured within an electrolyte a cer- 

 tain quantity of electricity traverses the electrolyte, which is the 

 same in all cases. This definite quantity of electricity I shall 

 call E P If we make this our unit quantity of electricity, we 

 shall probably have made a very important step in our study 

 of molecular phenomena. 



Hence we have very good reason to suppose that in Y 1? Gi, 

 and E x we have three of a series of systematic units that in an 

 eminent sense are the units of nature, and stand in an intimate 

 relation with the work which goes on in her mighty laboratory. 



6. The approximate values of V 1 and G x are known ; and I 

 will presently endeavour to evaluate E x . Y x has been variously 

 determined by experiment as 3*10 metre-eights per second, 

 2 '82 metre-eights per second, 2*88 metre-eights per second, 

 and may be assumed to be not far from 3 metre-eights per 

 second. Accordingly we may put 



Yi = 3 YIII metres per second. . . . (1) 



Similarly, if we use the value given by Sir John Herschel 

 for the mass of the earth, viz. 5942 XYIII* English tons, 

 which = XXIY grammes, we find that 



Gi= ixinM (2) 



7. To determine E 1? we must first establish a relation be- 

 tween the gaseous molecule of a body and what in chemistry 

 is called its atom. To do this, let us start with the definition 

 that a chemical atom is the smallest mass of each kind of pon- 

 derable matter that has been found to enter or leave a combi- 

 nation. Now, from Boyle and Charles's law we know that in 

 all gases there are approximately the same number of mole- 

 cules per litre, if they be taken at the same temperature and 

 pressure ; from experiments on diffusion we know that these 

 molecules are alike in mass; and from the phenomena of che- 

 mistry we know that they are alike in other respects. 



* The Roman fipnires following a number stand for cyphers. Thus, 

 3 VIII signifies 3 X 10 s , and 5942 XVIII stands for eighteen cyphers fol- 

 lowing 5942. Where no number precedes the Roman figures the number 

 1 is to be understood ; so that in XXIV grammes, the Roman figures stand 

 for 1 followed by 24 cyphers, in other words, for 10 24 , a number which 

 may conveniently be called the unit- twenty-four. 



