370 Dr. T. Carnelley on the 



Sb compounds, the atomic weight of As being less than that 

 of Sb. Again 



Melting-point. Melting-point. 



SnBr 4 = 303 

 SiBr 4 =260 



43 



Snl 4 =419 

 Sil 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 than 

 in that of the iodides, i. e. where the atomic weight of the 

 negative element is least. 



(4) The melting- or boiling-point or heat of formation of a 

 bromide is always nearer to that of the corresponding chloride 

 than to that of the corresponding iodide ; and the melting- or 

 boiling-points of the halogen compounds of the middle member of 

 three consecutive elements of the same group are always nearer to 

 those of the first (i. e. the one with least atomic weight) than to 

 those of the last member. Thus : — 



SbCl 3 . SbBr 3 . Sbl 3 . 



Melting-point = 345 363 438 



Difference = ~18 75~ 



NaCl. 

 Melting-point = 1045 





NaBr. 

 981 



Nal. 



901 



Difference = 



PC1 3 . 

 Boiling-point = 351 



V 



64 



AsCl 3 . 

 405 



A 



80 



SbCl 3 

 496 



Difference = 



— V 



54 





91 



The former of these phenomena probably depends on the 

 fact that the atomic weight of Br is nearer to that of CI than 

 to that of I ; and the latter on the fact that the atomic weight 

 of the middle member of three consecutive elements of the 

 same group is always less than the mean of those of the other 

 two elements ; thus — 



CI. Br. I. 



Atomic weight = 35*5 80 127 



Difference = 44*5 47 



P. As. Sb. 



Atomic weight = 31 75 122 



Difference = 44 47 



_j 



We have here, therefore, a good instance of the influence 

 of atomic weight on the physical properties of compounds. 



