Prof. T. Carnelley on the Periodic Law. 13 



properties of the bromide are always less than the mean of 

 those of the chloride and iodide, thus : — 



SbCl 3 . SbBr 3 . Sbl 3 . 

 Melting-point = 345° 363° 438° 



Mean of the melting-points of the chloride and iodide 

 = 391°. 



Whereas, when they decrease from chlorides to iodides, the 

 physical properties of the bromide are always greater than the 

 mean of those of the chloride and iodide, thus : — 



NaCl. NaBr. Nal. 



Melting-point = 1045° 981° 901° 



Mean of the melting-points of the chloride and iodide 

 = 973°. 



(b) The melting- or boiling-points of the halogen com- 

 pounds of the middle member of three consecutive elements 

 of the same group (proved only for Groups III. to VII.) are 

 always less than the mean of those of the other two, thus : — 



PBr 8 . AsBr 3 . SbBr 3 . 

 Boiling-point = 351° 444° 575° 



Mean of the boiling-points of PJ3r 3 and SbBr 3 =463°. 



From the above it follows that : — 



(1) The melting-point, boiling-point, and heat of formation of 

 a bromide are alivays nearer to those of the chloride than to those 

 of the iodide, (2) The melting- and boiling-points of the halogen 

 compounds of the middle member of three consecutive elements 

 of thie same group are always nearer to those of the compounds 

 of the first member (i. e. the one with the lowest atomic weight) 

 than to those of the last member. This probably depends on 

 the fact that the atomic weight of Br is less than the mean of 

 those of CI and I ; and 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 : — 



01= 35-5 i P = 31 C =12 | K= 39 



Sb = 122 



I = 127-0 

 Mean= 81-25 

 Br= 80 



Mean= 76*5 



As= 75 



Zr = 90 I Cs = 133 

 Mean = 51 Mean= 86 

 Ti=48 Pb= 85-5 



V. The melting-points of the halogen compounds of the 

 first and second groups are widely separated from those of 

 the other groups (see Relation 2 and Plate I.), there being, 

 in fact, a great jump from members of the third to those 



