172 Proceedings of Royal Society of Edinburgh. [sess. 
will simply obey Boyle’s law, provided that no further solution of 
the gas takes place and that no higher compound is formed. At 
any other constant temperature the horizontal part of the curve 
will be displaced upwards or downwards, parallel to itself, and the 
abrupt change of curvature will again take place at the same con- 
centration, C', of the gas in the solid, that is, when the whole of the 
solid has been converted into the compound. On removing gas from 
the system the same diagram will be traced in the reverse order. 
We need not here take account of complications which have 
been carefully considered by Hoitsema ( Zeits . f. physikal. Chem ., 
xvii. 1). It will be sufficient to recognise the fact that chemical 
compounds are characterised by a constant dissociation pressure, 
that the pressure changes abruptly when, and only when, the 
maximum quantity of the compound has been formed, and that 
alteration of temperature does not affect the point at which the 
abrupt change of pressure takes place. If solution of the gas takes 
place before combination begins, the left-hand end of the horizontal 
part of the diagram may undergo modification ; but this does not 
apply to the right-hand end, where the maximum quantity of the 
compound is formed. 
These rules have been amply verified experimentally in the 
case of salts containing water of crystallisation (Frowein, Zeits. /. 
physikal. Chem., i. 5 ; Andrese, Zeits. /. physikal. Chem., vii. 
