SECOND REPORT -1832. 
420 
Tlie third column exhibits the relation of the specific heats 
at constant pressures to that with constant volumes. 
On the specific heats of the four compound gases here given, 
M. Dulong observes, that as far as they go they are in agree¬ 
ment with the law before announced by him, namely, that there 
exists a very simple relation between the capacity of compound 
and elementary atoms. 
Dr. Prout has suggested an idea, which, if correct, would take 
away all force from the arguments which, as above stated, have 
been drawn in favour of some atomic weights, from the rela¬ 
tion supposed to exist between them and the specific heats. 
He contends that instead of combining always in the ratios in¬ 
dicated by the received atomic weights, bodies may combine in 
many multiples and submultiples of those ratios, constituting a 
series of combining numbers. Thus water, the atomic weight 
of which as generally received is 9, may combine in quantities 
represented by 3, 6, 9, 12 *, This involves a principle 
similar to that of Dumas, who represents atoms as made up of 
groups of chemical molecules f. 
Doctrine of Volumes .—The doctrine of volumes is another 
point on which chemists are not agreed. The difference which 
obtains among them has reference to the relation between the 
volumes and the atoms of the permanent gases and bodies capa¬ 
ble of being volatilized. This relation is stated in chemical works 
in three different ways. The first, that of Ampere and Dumas, 
presents it in its simplest form. According to them, the atom 
and the volume are the same in all gases and vapours ; or equal 
volumes of all gases, simple or compound, contain exactly the 
same number of atoms. This opinion is founded chiefly on the law 
of Mariotte, that the volume of any gas whatsoever is inversely 
as the pressure applied to it. For, it is argued, were the number 
of atoms in each gas not the same, the law of the diminution of 
volume under pressure ought to vary for each gas. But either 
this law is not correct beyond certain limits which experiment 
has not yet reached, or it is independent of the quantity of 
matter which the gaseous substance contains;—for carbonic 
oxide unites to half its volume of oxygen to form carbonic acid 
without increase of bulk, and yet both gases obey the same law 
of compression. Besides, the adoption of this simple relation 
between atoms and volumes leads to considerable difficulties, 
when we examine the constitution of the compound gases. 
If, for example, one volume of hydrogen be combined with 
* Daubeny’s Atomic Chemistry. 
f Traite, tom. i. p. 40. 
