" 



432 Mr. S. U. Pickering on the 



The number of more or less sudden changes of curvature 

 noticed amounted to seventeen, and, of the eight separate 

 series of determinations, 



1 showed 1 of the changes, 



2 



» 



1 



91 



?J 



3 



v 



1 



11 



91 



4 



?j 



4 



>> 



99 



5 



?• 



1 



99 



?> 



6 



j? 



7 



99 



J? 



7 



jj 



2 



•) 



?J 



and it may be fairly stated that in no case did any of the 

 series fail to show a change except through lack of sufficient 

 data. 



The concordance, moreover, of the actual position of the 

 changes shown by the various properties was certainly better 

 than might have been expected, considering the great difficulty 

 in determining the exact point at which any two constituent 

 curves cut or touched each other. The average difference 

 between their position as shown by the different individual 

 properties (or by the same property at different temperatures) 

 and by the mean results was only 0*388 per cent., and in only 

 nine cases did it reach or exceed 1 per cent. 



Another point which adds confirmation to these results is 

 that in every case where the hydrate indicated is sufficiently 

 simple to admit of its composition being determined accu- 

 rately (that is, where a difference of 2 to 11 per cent, wouid 

 be caused by the addition of one more water-molecule), tfye 

 changes occur at a point corresponding, within the possible 

 limits of error, to a definite molecular composition*. There 

 are seven such cases available, and the found and calculated 

 values show an average difference of only 0226 per cent, or 

 about 0-057 H 2 ; the maximum reaching but 0'48 per cent 

 or T^-th H 2 0. 



A comparison of the density-results at the four different 

 temperatures is very instructive. The first differentials show 

 gradual changes in their general character as the temperature 

 alters, the curvature in the various portions becoming more 

 marked as the temperature is lower ; yet, in spite of these 

 alterations in general appearance, the points at which definite 

 alterations in curvature occur remain unaltered f . The con- 



* Prof. Arrhenius implies that they do not; but as my found values 

 were not published at the time when he wrote, I do not see how he was 

 in a position to judge. 



t In attacking Mendeleeff, Prof. Arrhenius falls into the error of con- 

 founding changes in general direction, especially positions of maximum 



