of Solid Substances. 343 



cold must be the result of their action. In special experiments, 

 made with the object of determining the thermal action, neither 

 absorption nor evolution of heat could be detected. Thus the 

 formation of iodate of potash is attended with an evolution of 

 heat. This would then accord with the easy transformation of 

 the chlorates into iodates, or of chloric acid into iodic acid, and 

 the easy transformation of the iodide of potassium into the iodate, 

 through the action of permanganate of potash, seeing that we 

 must have an evolution of heat. The oxygen, therefore, may be 

 assumed to be in a very different condition relatively to the other 

 elements ; or else we must suppose that it has not affected the 

 coefficient of contraction, certainly not to have diminished it. 

 The author throws out this simply as a possible explanation ; 

 he is well aware that many other explanations 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. Ste. -Claire Deville has given an explana- 

 tion 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 con- 

 stituents 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 associa- 

 ting this increase of volume with 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 freez- 

 ing-point of water, and Mousson has actually liquefied ice bv 

 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 decompose a chemical compound, if the forma- 

 tion 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 amal- 

 gams seems to confirm this anticipation. Joule has shown that 

 the amalgams of zinc, lead, and tin are decomposed by pressure 

 alone ; and these are the amalgams produced with the least con- 



