422 
SECOND REPORT— 1832. 
Sp. grav. 
Mercury, vapour = 6*976 .*. atom, weight = 6*327* 
Phosphorus, vapour = 4*42 .*. --= 4*005 
Sulphur, vapour = 6*667 .*.-= 6*0012 
These results give for the composition of 
Protoxide of mercury 4 atoms mercury + 1 atom oxygen. 
Phosphoric acid. ... 1 atom phosphorus + 5 atoms oxygen. 
Sulphuric acid .... 1 atom sulphur + 9 atoms oxygen. 
To account for the difference between the atomic weights 
of mercury and sulphur obtained by this process, which if true 
would introduce such striking changes into the received theo¬ 
retical composition of their compounds with oxygen and the 
other electro-negative elements, Berzelius supposes, with Du¬ 
mas, that in the less volatile bodies the number of atoms con¬ 
tained in a given volume of their vapour may be less than in 
an equal volume of bodies so easily converted into vapour as 
to constitute permanent gases at the usual temperature and 
pressure of the atmosphere. The subject is one of great in¬ 
terest, and further experiments can alone clear up the obscuri¬ 
ties which at present invest it. 
The valuable results published by Dumas in 1827 (Ami. de 
Chim. xxxiii. p. 337), exhibiting the density of iodine, mercury, 
the chlorides of phosphorus, arsenic, silicon, boron, tin and 
titanium, and the fluorides of silicon and boron in the gaseous 
state by direct experiment, and those of phosphorus, arsenic, 
silicon, boron, tin and titanium by inference, are only exceeded 
in interest and importance by those above stated for sulphur and 
phosphorus. 
Isomorphism , use of \ in determining the compositions of ox- 
ides, fyc .—The doctrine of isomorphism has lately proved of 
much use in determining the true atomic constitution of many 
compound bodies. This application of it, though often made 
by Berzelius, is in a great measure unknown in this country. 
The law of isomorphism, as announced by Mitscherlich in its 
utmost generality, is as follows:—“The same number of atoms 
combined in the same way produces the same crystalline form, 
and the same crystalline form is independent of the chemical 
nature of the atoms, and is determined only by their number 
and relative position.”—This law has undergone a slight modi¬ 
fication f since it was promulgated by its distinguished author, 
but in no way to affect the conclusions to be drawn from it in 
regard to the atomic constitution of bodies. 
Now in applying the law to the composition of the metallic 
f Taking the sp. gr. of oxygen at 1T026. f See page 425. 
