250 
Proceedings of the Royal Society of Edinburgh. [Sess. 
H 
0 i 
Cl 
O 2 H 
° H 
p2 0 j 
^ 0 ) 
and C 2 qJ 
H 
o f 
Cl 
Here the whole four of hydrogen are not bound by a mutual affinity ; for each 
element of hydrogen can be substituted for one of chlorine in regular series, 
beginning with the first and ending with the last. The atoms of oxygen are, on the 
contrary, united in pairs (which will be more fully developed hereafter), and only 
for two atoms of oxygen two of chlorine can be substituted ; thus — 
01 
p 2 0 j 
O O { 
0 1 
C 2 
Cl 
Cl 
0 
0 
C 2 
Cl 
Cl 
Cl 
Cl 
In the same manner, with bodies that contain multiples of C 2 united to 
hydrogen, etc. 
Take the inverse of this. If the four atoms of hydrogen were bound together, 
we could evidently expect to form such bodies as 
Cl 
Br 
Br 
Br 
PI 
H 4 
11 Cl 
H 4 Cl 
Cl 
H 4 
11 Cl 
and H 4 !]'' 
Br 
Cl 
Cl 
Cl 
Cl 
or for bodies like C 4 H 4 , C 6 H 6 , C 8 H 8 , one would naturally expect to find the carbon 
Cl 2 
substituted for chlorine, and find bodies like H 4 0 „, H 6 C1 6 , II 8 C1 8 , etc. 
These bodies are not only unknown, but the whole history of hydrogen might be 
investigated and not a single instance be found to favour the opinion that it has any 
affinity for itself when in union with another element. 
How, on the other hand, carbon remains chemically united to carbon, 
while perhaps 8 equivalents of hydrogen are exchanged for 8 equivalents of 
chlorine, as in naphthaline. Analogous to this is the conversion of alcohol, 
C 4 ° 5 and the hydrocarbide C 4 H° into C 4 C1 6 . All the countless instances 
of substitution of chlorine, etc., tend in the same direction. They prove beyond 
doubt that carbon enters into chemical union with carbon, and that in the most 
stable manner. This affinity, one of the strongest that carbon displays, is perhaps 
only inferior to that which it possesses for oxygen. 
Another feature in the affinity of carbon is, that it combines by degrees of 
two; thus, C 2 0 2 and C 2 0 4 , C 4 H 4 and C 4 H 6 , C 6 H 8 and C 6 H 8 , C 8 H 8 and C 8 H 10 , 
etc. : from these last it is especially evident that two is the combining grade of 
carbon. It becomes still more apparent when we compare the bodies C 4 H 4 and 
C1 1 
C 4 H 5 C1, that is, C 4 H I etc. Many such proofs might be added, while, on the 
H 4 
* Misprinted “ C 2 ” in the original. See the French paper. 
