Laws of Molecular Force. 19 



it is reasonable to imagine that in the last equation 1 — E^j, 

 1— E 2 /i 2 and so on, can be replaced by a single mean value 

 proportional to bd, where b is the linear coefficient of expan- 

 sion of the solid compound. Denote each by abd, where a is 

 a constant which may or may not be the same as the 7 for 

 metals, and replace w 1 ra 1 v, 2 4-7i 2 m 2 t'2 2 4- . . ., which is twice the 

 total kinetic energy of the molecule, by its value 2JMc0, 

 where c is the specific heat of the compound, then the last 

 equation becomes 



2JMc 1 v 



w - F =-tr< i >{r), (12) 



in which b is at present unknown for most compounds. But 

 as in the metals 5TM r ='044, so we may assume for com- 

 pounds that 6TM^ is constant ; and merging this unknown 

 constant and 1/a into a single constant k, for compounds of 

 the same type we get finally 



Mr 



M 2 Z=5-8xlO- 4 ^(M/ /0 )TM^, . . . (13) 



differing from the equation (9) for elements only in the con- 

 stant k to be ascertained for each type of compound. It so 

 happens that for binary compounds of monad elements such 

 as NaCl or KI, the value 1/2 for k gives good results ; and as 

 Mc for the chlorides of this type is 12'7, and for the bromides 

 13 8, and the iodides 13*4, the mean of which can be taken 

 as practically double the 6*4 for the atomic specific heat of 

 the elements, then the equation (13) for such compounds 

 reduces to the same as that for the elements (9). 



We will first take the data for the haloid compounds of the 

 metals of the Li family ; the melting-points are those given 

 in Carnelley's paper on the Periodic Law (Phil. Mag. ser. 5, 

 xviii., also Journ. Chem. Soc. xxix., xxxiii., xxxv., xxxvii.) ; 

 the molecular domains (volumes) are those given by F. W. 

 Clarke (Phil. Mag. ser. 5, iii.), and differ slightly from those 

 previously given in this paper in Table IX. Below the 

 values of (M 2 /) 5 calculated by the equation (9) are given 

 values from Table IX. found from surface-tensions. 



Table XIII. 

 Li. 



F. CI. Br. I. 



T 1070 870 820 719 



M/p 11-3 21-3 280 384 



M 2 J 120 20 280 36-2 



(M 2 3-5 4-5 5-3 60 4 6 57 64 



{H/Ptft &cm \ 4 .o 5.0 fi . 



labia IX. f •• 4d DJ bU 



C2 





Na. 







F. 



OI. 



Br. 



I. 



1270 



1045 



981 



901 



164 



273 



334 



435 



21-2 



325 



41-1 



524 



