344 On the Atomic Volume of Solid Substances. 



traction of any. In order to get definite proof of 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 experi- 

 ments, 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). 





Specific 

 gravity. 



Calculated 

 specific gravity. 



V+V 



V 



Pb 2 Hg 



11-979 

 12484 

 12-815 



12008 

 12-358 

 12-734 



10024 

 0-9899 

 0-9937 



PbHg 



PbHg 2 





The specific gravity of the mercury used in calculating the mean 

 density was 135 73. 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 amal- 

 gam was 100 of mercury to 51*01 of tin, and the specific gra- 

 vity 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 chan- 

 ging 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 5*2, was sometimes 

 double this amount, or even more. It has also been remarked 

 that if the metal in combining were to expand, the volume of the 

 oxygen would appear as a maximum. This apparently large 

 volume of the oxygen seems to belong to suboxides, such as sub- 

 oxides of mercury and copper, and oxide of silver. If we sup- 

 pose, 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 possible that mere pressure would decompose these 

 oxides, at least in part, into metal and the higher oxide. The 

 instability of a body of this type, such as suboxide of mercury, 

 is well known, mere trituration effecting the liberation of metal 

 with formation of the higher oxide. In this way, therefore, it 

 seems to support the argument adduced. 



