Double-Layer of Solid and Liquid Bodies. 305 



simple bodies, consisting oil single atoms, we immediately 

 conclude that (1) the intrinsic potentials of metals are always 

 positive. 



This result corresponds to the sign ■+- in formula (8) ; it 

 is, as pointed out in § 1, in complete agreement with the 

 facts. 



If we suppose that the atomic radii (r) are exactly equal 



to half the mean interatomic distance R= — ^^, then, 



1 2^n' 



putting r=U and w=™, we may replace (8) by the 



following approximate formula : 



V= + jg (10) 



We notice that the right-hand member of this equation is 



of the same order of magnitude as the ionizing potential of 



1 /ce 2 

 the metal, since - -p- is approximately equal to the energy 



required to remove one of the k electrons to infinity : hence 

 (2) the intrinsic potentials of metals are of the same order of 

 magnitude as the corresponding ionizing ■potentials. 



It always seemed strange to me that the units invented 

 for the measurement of contact electromotive forces (i. e. 

 the differences between the intrinsic potentials) should fit so 

 well the ionizing potentials, which, indeed, in the case of 

 metallic vapours amount to a few volts. On the above theory 

 this coincidence receives a very simple and natural explanation. 



Lastly, since V is proportional to k, and since the atomic 

 radii r are, probably, less variable than the valency, we see 

 that (3) the intrinsic potentials of metals tend to increase with 

 their valency. 



In fact, it is well known that the alkali metals are the 

 only ones sensitive to ordinary light : next come the di- 

 valent ones (Zn, Mg), &c. ; on the other hand, Pt [k = S) is 

 one of the most electronegative metals *. 



A few examples are collected in the following Table (I.), 

 the intrinsic potentials (column V.) being calculated on 

 formula (10) ; the atomic volumes (equal to the ratio of the 

 atomic weight A to the specific weight S, column III.) have 

 been used for the determination of n, by means of the relation 



n= t-N, where N = G'0G x 10 23 is Avogadro's number. 



* We shall further see (§ 6) that this is "but very approximately true 

 for pure metals unaffected by absorbed leases. For instance, potassium 

 purified by distillation ceases to be sensitive to ordinary light. 



