NOEMAL SULPHATES OF POTASSIUM, RUBIDIUM, AND CESIUM. 
489 
together with the fact that the increment 26 is not determinable with the accuracy of 
the constant a, the coefficient at 0°, suggests the probability that the three lines 
should all cross at one point, somewhere near 136°. At this temperature the value 
for rubidium sulphate only exhibits a difference of one in five hundred from the two 
identical values for the two other salts, so that the three values are identical within 
the limits of experimental error. 
Graphical Expression of Cubical Expansions. 
These considerations may be summarised in the following addition to the last 
italicised statement. 
In consequence of this fact the coefficients of cubical expansion of the three salts 
converge loith rise of temqjerature toivards equality, which, within the limits of 
experimental error, they reach at 136°. Beyond the temperature at which identity 
of expansion occurs the coefficients of expansion exhilnt increasing divergence, the 
order of progression being inverted, an increase in the atomic weight of the metal 
being now accompanied by an increase in the coefficient of cubical expansion. 
The Linear Exptansion. 
The first conclusions to be drawn from an inspection of the coefficients of linear 
deformation are the following : 
The thermal deformation is of the nature of an expansion along all directions 
in the crystals of the three sulphates, no contraction occurring in any direction. 
3 R 
VOL. CXCII.-A. 
