75 
of Edinburgh, Session 1869-70. 
The oxygen, therefore, may he assumed to he in a very different 
condition relatively to the other elements, or else we must suppose 
that it has not affected the co-efficient of contraction, certainly not 
to have diminished it. The author throws out this simply as a 
possible explanation ; he is also well aware that many other ex- 
planations might be given, all, possibly, equally satisfactory. "But 
a physical explanation, however far it may lie from the truth, 
seems to convey to us the clearest ideas of what may possibly take 
place. 
There is one point connected with the subject of volumes that 
requires very careful attention. All bodies in combining do not 
unite with condensation ; that is, the volume of the compound 
might exceed the volumes of the isolated constituents, and yet a 
large evolution of heat might take place during its formation. A 
well-known example is that of iodide of silver. Now, M. Fizeau 
has shown that iodide of silver contracts regularly with increase of 
temperature, and M. St Claire Deville has given an explanation of 
this anomaly. Deville believes that bodies combine at such a 
temperature as would be required to transform the volume of the 
compound to that of the sum of the volumes of its constituents in 
the free state. Applying this to iodide of silver, it is clear that 
contraction must take place, and in all similar cases where we have 
an increase of volume. One cannot help associating this increase 
of volume to a purely physical change of state, such as the change 
of water with expansion into ice. Now, as Sir William Thomson 
has proven that pressure lowers the freezing point of water, and 
Mousson has actually liquefied ice by enormous pressure, if the 
formation of a chemical compound is analogous to a physical 
change of state, we ought to be able by mere pressure to decom- 
pose a chemical compound, if the formation of that compound is 
attended with an increase of volume. No doubt, in order to get 
experimental proof of this fact, we must use a relatively weak 
chemical compound, one attended with the evolution of no great 
amount of heat; and the well-known experiments of Joule on the 
effect of pressure on amalgams, seems to confirm my anticipation. 
Joule has shown that the amalgams of zinc, lead, and tin are de- 
composed by pressure alone, and these are the amalgams produced 
with the least contraction of any. In order to get definite proof of 
