12 Prof. T. Carnelley on the Periodic Law. 



of the elements on the melting '- or boiling-point increases with its 

 own atomic weight and decreases with the atomic weight of the 

 other element. Thus : — 



Melting-point, j Melting-point. 



PBr 3 = 252° PBr 3 = 252 



AsBr 3 = 295 | PI 3 = 328 



In each of the above pairs of compounds, which contain 

 one element in common, it is seen that the melting-point 

 increases with the atomic weight of the variable element, and 

 that whether the latter be the positive or negative constituent, 

 thus showing that the influence of an element on the melting- 

 point of one of its compounds increases with its own atomic 

 weight. The fact that the influence of an element on the 

 melting-point or boiling-point of its compounds increases with 

 its own atomic weight has been known for a long time with 

 regard to some few series of compounds ; but I am not aware 

 that it has been pointed out as of general application to all 

 the elements of Groups III. to VII. For the even members 

 of Groups I. and II. and for Na and Mg the influence is 

 always of the opposite kind. 



That the influence decreases with the atomic weight of the 

 other element is shown by the following examples : — 



Melting-point. 



PBr 3 = 252 



PC1 3 = 200 



52 



Melting-point. 



SbBr s = 363 

 SbCl 3 = 345 



18 



Here the substitution of Br for CI produces a larger increase 

 in the melting-point in the case of the P than in that of the 

 Sb compound, the atomic weight of P being less than that 

 of Sb. 



Again : — 



Melting-point. 



SnBr 4 = 303 

 biBr 4 = 260 



~43 



Melting-point. 

 Snl 4 = 4X9 

 SiI 4 = 393 

 ~26 



As before, the substitution of Sn for Si produces a greater 

 influence on the melting-point in the case of the bromides, 

 i. e. where the atomic weight of the negative element is the 

 least. (Compare also Relations 3, 4, 7, 8, and 9.) 



Conclusion III. applies only to Groups III. to VII. 



IV. (a) Where the melting- or boiling-point or heat of 

 formation increases from chlorides to iodides, these physical 



