7(5 Proceedings of the Royal Society 
the expansion, it is, of course, necessary to use the specific gravity 
of mercury in the solid state. Now, Joule states, as the mean of 
his experiments, that mercury in the solid form has the specific 
gravity 15-19, whereas in the above amalgams it would have the 
density of only 14*1. The observations of Matthiessen on the 
specific gravity of alloys enables us to confirm Joule’s results : — 
Lead Series (A. Matthiessen). 
Sp. Or. 
Calculated 
Sp. Gr. 
V + V' 
V 
Pb 2 Hg, . . . 
11-979 
12-008 
1-0024 
PbHg, . . . 
12-484 
12-358 
0-9899 
PbHg,, . . . 
12-815 
12-734 
0-9937 
The specific gravity of the mercury used in calculating the mean 
density was 13-573. Now, seeing that there is little or no con- 
traction, and even in one case a slight expansion, in taking the 
above specific gravity of mercury, the higher density of mercury 
given by Joule as the result of his experiments would necessarily 
lead to an expansion in their formation. To illustrate the effect 
of pressure on the composition of an amalgam, let us take Joule’s 
experiments on the tin amalgam. The composition of this 
amalgam was 100 of mercury to 51-01 of tin, and the specific 
gravity 10-518. The effect of 5400 lbs. pressure for thirty days, 
changed the amalgam, so that it had ultimately the composition 
100 of mercury to 384 of tin. It is natural to believe, therefore, 
that the effect of pressure in this case is quite analogous to the 
inverse change of state, when a body that has expanded in chang- 
ing its state has been subjected to its influence. 
In the early part of this paper we saw that the volume of oxygen 
in some oxides, instead of being 52, was sometimes double this 
amount, or even more. It has also been remarked, that if the 
metal in combining was to expand, the volume of the oxygen 
would appear as a maximum. This apparently large volume of 
the oxygen seems to belong to suh-oxides, such as sub-oxides of 
mercury and copper, and oxide of silver. If we suppose, now, that 
this large increase of volume in the oxygen is the result of an 
expansion in the metal in combining with the normal oxide, it is 
