Dissociation versus Hydration. 37 



function (d 2 s/dp 2 ) consisted of straight lines. Mathematically 

 interpreted, this means that in the ds/dp curve angular points 

 or sudden changes of curvature occur. If Mr. Pickering had 

 " smoothed " his curve properly he would evidently have 

 removed these angular points or sudden changes of curvature, 

 for a very small fraction of the " experimental error " would 

 suffice for this purpose. The result can scarcely be gratifying 

 to the supporters of the theory of hydration. Mr. Pickering 

 finds that d 2 s/dp 2 is made up of no less than 17 straight lines 

 corresponding to 16 hydrates. In other words, the specific 

 gravity can be represented in the form of 17 equations of the 

 third degree with 68 arbitrary constants, besides the 16 arbi- 

 trarily chosen points where the curves begin and end ! 



This really has very much the look of a reductio ad absurdum. 

 The mode of representation entirely lacks experimental founda- 

 tion, as Mr. Pickering himself tacitly admits in the words 

 " owing to the magnitude of the experimental error." It is 

 characteristic also that Mr. Pickering "agrees with Mr. 

 Crompton's conclusion that they (the d?k/dp 2 curves ; k = con- 

 ductivity, p = per cent, by weight of sulphuric acid) give a 

 rectilineal figure, but he differs from him in some of the 

 details as to where the breaks occur" (p. 88). But the 

 points " where the breaks occur" should correspond to definite 

 hydrates. The fact is that Mr. Pickering with his multitu- 

 dinous arbitrary constants can Hx the points " where the 

 breaks occur " just where he chooses, and so we need not 

 wonder that the curve for d 2 k/dp 2 can be drawn in such a 

 manner "that these breaks agree very closely with those 

 shown by his own density-results " (p. 88). 



I will quote in addition a very instructive statement of 

 Mr. CrompWs (Proc. Chem. Soc. Dec. 1888, p. 127) :— " Mr. 

 Crompton, replying to Dr. Morley's objection that there did 

 not seem to be any reason why a limit should be put to the 

 differentiation when that had been performed twice, and that 

 it would be just as reasonable to proceed with a third or 

 fourth differentiation and so on, said that a limit to the dif- 

 ferentiation would necessarily have to be made according to 

 the nature of the case under investigation and the discretion 

 exercised by the investigator. In the present instance the limit 

 of differentiation is clearly indicated by the agreement of the 

 results obtained with those previously arrived at by MendelejefF 

 by discussing a totally different physical property." But now 

 that Prof. MendelejefFs results are proved to be " erroneous," 

 we should perhaps expect that the differentiation ought to be 

 carried a little further. This, however, is not necessary, as 

 most of the physical properties can only be determined with 



