Influence of Atomic Weight. 371 



(5) The melting- and boiling-points of the halogen com- 

 pounds of the elements belonging to the first and second 

 groups of Mendeljeff's classification are widely separated from 

 those of the other groups, being in fact considerably higher. 

 Different relations, too, often appear to exist between the melt- 

 ing-points of even members of these two groups from those 

 which exist between groups (3-7) inclusive ; while the com- 

 pounds of the elements which are often placed in the odd divi- 

 sions of the first and second groups are generally altogether 

 irregular. In the case of the odd members of the first group, 

 this may be explained by the fact that it is very uncertain 

 whether Cu, Ag, and Au really belong to the same group as 

 Na or not, as pointed out by Mendeljeff in his memoir on the 

 Periodic Law ; for he places these elements not in the first 

 group along with Na, but in the eighth with Fe, Pd, Pt, &c. 



In the paper above referred to it has been shown how the 

 preceding relations may be applied 



(1) To the calculation of unknown melting- and boiling - 

 points. — The following are instances of melting-points pre- 

 dicted by this method having been verified by experiment : — 



Predicted. Experiment. 



CsCl Below 959 904 \ * 



CuBr 782 777/ 



BeCl 2 (820-870) (858-890) 



BeBr 2 (802-820) (858-890) 



(2) To the determination of the atomic weights of elements 

 when the application of the methods of specific heat and 

 vapour-density are inadmissible or uncertain. By this means 

 the atomic weight of Be has been found to be 9*2, thus 

 agreeing with the determination of the specific heat made by 

 Emmerson Reynolds, and proving Nilsson and Pettersen's 

 determination to be incorrect ; for they found a value for the 

 specific heat corresponding to the atomic weight 13*8. 



As far as existing data allow us to judge, the compounds of 

 the elements with fluorine and also with monatomic organic 

 radicals, likewise obey the same laws as those of the halogen 

 compounds. The fluorides, however, cannot be strictly com- 

 pared with the chlorides, bromides, and iodides, since F is an 

 even member of the seventh group ; whereas CI, Br, and I are 

 odd members. 



Boiling-points of Organic Compounds in general. — The 

 boiling-points and melting-points of carbon compounds de- 

 pend : — (1) On the atomic weights or nature of the consti- 



* These two numbers have not been previously published. 



