Dissociation versus Hydratit 



33 



plaining the facts was abandoned. Similar attempts have 

 been made frequently of late, as may be seen by looking into 

 almost any book on thermochemistry, where we find such 

 curves for thermal data. It is obvious that in any not too 

 simple curve singular points will occur. The conclusion is 

 that if we look in this way in such a curve for evidence of the 

 existence of hydrates we shall certainly find it, for every pro- 

 perty can be represented by a curve which is usually not very 

 simple (and were it by chance simple, wide conclusions might 

 yet be drawn from it) . The peculiar character of such con- 

 clusions is that no premisses are required for them. 



Prof. Mendelejeff has been very unhappy in his choice of a 

 property to prove the existence of hydrates. The reasons 

 which Ostwald has given, that u specific gravity cannot well 

 be used for setting forth stochi'ometric laws," must be con- 

 sidered correct. From the curves which represent the first 

 derived functions of the specific gravity as a function of the 

 percentage composition by weight, Prof. Mendelejeff seeks to 

 deduce the existence and composition of hydrates. This curve 

 for solutions of sulphuric acid, which is given as being par- 

 ticularly instructive, has been twice plotted by Mendelejeff. 

 Below we reproduce the first form it assumes (Ber. deut. chem. 

 Ges. 1886, p. 386). 



Fiar. 1. 



H,SO, 



In this figure the empirically obtained numbers are repre- 

 sented. In the second curve (see fig. 2) which is influenced 

 by theory (Zeksclir . fur physikal. Chemie, i. p. 275) the same 

 numbers are represented in a totally different manner. 



Who would be likely to discover that these two curves are 

 identical ? Who could recognize the first curve in the straight 



Phil Mag. S. 5. Vol. 28. No. 170. July 1889. D 



