xvi ATOMIC WEIGHTS OF THE METALS 367 



Molecular heat of compounds Neumann, in 1831 (Pogg, 

 Ann. der Physik, 1831, 23, 1-39), found a constant value 

 for the MOLECULAR HEATS (sp. ht. x mol. wt.) of a series of 

 seven carbonates and another constant value for a series of 

 four sulphates ; in each of these cases the atomic heat of 

 the combined metals was constant throughout the series. 



Cannizzaro, in 1858 (A.C.R. XVIII. 23), showed that mer- 

 cury and iodine obey Dulong and Petit's Law when combined 

 together, and used this fact as a further proof of his view that 

 the atomic weight of mercury was 200 and not 100, as had 

 been supposed previously. The figures he quotes are as 

 follows : 



Atomic heats : 



Solid bromine, Br 1 80 x 0-0843 6746 



Iodine, I 1 127x0-0541= 6*873 



Solid mercury, Hg 200x0*0324 = 6*482 



Molecular heats : 



Mercurous chloride, HgCl 1 (2004- 35-5) xo*o52i = 12-258 = 2 x 6-129 

 Mercurous iodide, Hgl 1 .., (200+127 ) xo*O395= 12*913 = 2 x 6 457 

 Mercuric chloride, HgCl 2 . (200 + 71 ) x 0*0689 = 18*669 = 3 x 6*223 

 Mercuric iodide, HgI 2 ...(200 + 254 ) x 0-0420= 19*054 = 3 x 6*351 



If the atomic weight of mercury were 100, the molecular 

 heats of the mercurous salts should have been 3x6, and those 

 of the mercuric salts 4 x 6, for the quantities shown in the 

 table. 



A more detailed study of the molecular heats of com- 

 pounds was made in 1865 by Kopp ("Investigations of the 

 Specific Heat of Solid Bodies," Phil. Trans., 1865, 155, 

 71-202), who showed that "Each element has the same 

 specific heat in its solid free state and in its solid com- 

 pounds " (loc. cit. p. 83). 



Whilst Cannizzaro assumed that in all cases the 

 molecular heat of a compound '= number of atoms x 6, 



1 The argument is not affected if these formulae are written Br. 2 , I 2 , 

 Hg 2 Q 2 , and HgJ 2 - 



