Laws of Molecular Force. 25 



not 2 and 3 and 4 times the value of I— CI already found in 



connexion with the monad metals : we have the following 

 mean values: — 



I -CI. I 2 -Cl a . I3-CI3. I 4 -01 4 . 



1-4 1-8 2-7 3-1 



The discrepancies in these values might be ascribed to an 

 error in the assumption that £Mc/6*4 is equal to the same 

 constant 1 for all these types of compounds ; and it might be 

 supposed that the constant ought in each case to be chosen 

 so that I 2 — Cl 5 , I 3 — Cl 3 , and I 4 — Cl 4 are 2 and 3 and 4 times 

 I — 01. But this supposition breaks down when it is re- 

 membered that in the types RC1 3 and RC1 4 values of (M 2 Z)* 

 as found by the independent boiling-point method are only 

 8 per cent, larger than those by the other method. It is 

 true that the boiling-point method is only approximate, but 

 that the approximation is fairly close can be seen by com- 

 paring the following pairs of values, the first of each pair 

 being the approximate value from Table XV. and the second 

 the value from Table VII., namely : — 



PCI3. AsCl 3 . CC1 4 . SiCl 4 . Sn01 4 . 



6-0 6*3 6-3 6-6 7*4 



6-6 7-0 6-8 6-9 7'8 



These comparisons show that the phenomenon that the 

 values in (M 2 /)* for I -CI, 1 2 -C1 2 , I 3 -C1 3 , and I 4 -CJ 4 do 

 not stand as 1, 2, 3, 4 is a real one, and not the result of 

 accumulated imperfections in the methods of calculation. 

 The values given above for these differences stand more 

 nearly in the relation 1, 2% 3% 4*; and supposing it to be the 

 true one, they can be brought into almost complete harmony 



if we return to the old relation bTMe = constant, and denoting 



1 ° 



by E an equivalent replace it by 6TEs = constant, which would 



reduce the values of (M 2 Q* for the four types in Tables XIII., 

 XIV., and XV. by factors 1, 1/2", 1/3?*, 1/4A : then the 

 numbers 1*4, 1*8, 2*7, and 3*1 for I — CI, I 2 — Cl 2 , I 3 — Cl 3 , and 

 I4— Cl 4 ought to be as 1, 1/2 * 7 *, 1/3^, and 1/4^ . Dividing the 

 numbers by these powers, we get for I — CI the series of values 

 1*4, 1*2, 1*4, and 1*4, which are as nearly constant as possible 

 under the circumstances. 



This constitutes the main part of the proof that in com- 

 pounds of the type RS„, where R is an /i-valent atom and 

 S a monad, (M 2 /)* is of the form (F r + nF 8 )/?i*, whence 

 {Wl) b /?i h =F r /}i+F s . Thus our best plan will be to divide 

 the numbers in Table XIII. by 2~r?, and those in XIV. bv 



