532 CHANGES IN CHEMICAL NOTATION AND ATOMIC WEIGHTS. 
Oxide of Silver. Peroxide of Hydrogen. Silver. Water. Oxygen. 
Ag 2 0 + H 2 0 2 = Ag 2 + H 2 0 -f- G 2 
Hypochlorite of Barium. Peroxide of Barium. Water. Chloride of Barium. Hvdrate of Barium. 
BaCJ 2 0 2 + 2BaO s + 2H 2 0 = BaC] 2 + 2BaH 2 0 2 
Oxygen. 
+ 20 2 
There is one compound, too, of singular interest; hydride of copper, 
CuH 2 , acted upon by hydrochloric acid, gives an abundant evolution of hy¬ 
drogen gas. 
CuH 2 + 2HC1 == CuCl 2 + 2H 2 . 
Now, metallic copper alone does not decompose hydrochloric acid, but 
when to its affinity for the chlorine is added that of the hydrogen for hydrogen, 
the result is the reaction expressed in the foregoing equation. Other support 
is drawn by the advocates of these views from the analogy which exists be¬ 
tween the reactions of zinc with hydrochloric or hydriodic acid, and with 
the iodides of the alcohol radicles. Thus— 
Iodide of Ethyl. Ethyl. 
Z n + 2C 2 H 5 I = Znl 2 + (C 2 H 5 ) 2 
Hydriodic Acid. Hydrogen. 
Zn + 2HI = Znl 2 -f H 2 . 
There can be no doubt, for many reasons, that the molecule of ethyl is really 
constituted of two atoms and occupies two volumes. Hydrogen by analogy 
would seem to do so likewise. 
There is yet another experimental method to which appeal may be made in 
the determination of atomic weights. 
Dulong and Petit, in 1819, discovered the interesting law which bears their 
name. It is expressed thus :—The specific heats of simple bodies are in¬ 
versely as their atomic weights. The number representing the specific heat, 
therefore, multiplied by the atomic weight gives a constant product. The 
following table shows that, in order that these relations may be preserved, 
the new atomic weights must be employed in the case of those elements 
which have already been indicated as forming exceptions to the laws which 
regulate gas-volumes, and whose atomic values have therefore undergone 
revision :— 
Specific Heat 
of Equal Weights. 
Atomic Weight. 
Specific Heat 
x Atomic Wt 
Sulphur . . . 
. . OT776 
32 
5 6832 
Tellurium . . . 
. . 00474 
129 
6T146 
Magnesium . . 
. . 0-2499 
24 
5-9976 
Zinc. 
. . 0-0955 
65 
6-2075 
Cadmium . . . 
. . 00567 
112 
6-3504 
Iron. 
. . 0-1138 
56 
6-3728 
Mercury [solid] . 
. . 0-0319 
200 
6-3800 
Potassium . . . 
. . 0-1696 
39 
6-6144 
Iodine .... 
. . 00541 
127 
68707 
Bismuth . . 
. . 0-0308 
210 
6-4680 
Silver .... 
. . 0-0570 
108 
6T560 
Carbon— 
Graphite . . . 
. . 0-2000 
12 
2-4000 
Diamond . . . 
. . 0-1470 
12 
1-7640 
By examination of this table it will be seen that the product of the specific 
heat by the atomic w r eight is very nearly the same for all the elementary bodies. 
