THE HYDRATE THEORY OF SOLUTIONS 127 



It will be convenient to describe the gradual development of 

 the hydrate theory in two sections, noting in the present com- 

 munication the introduction into the theory of the ideas of mass 

 action and reversibility, and in a subsequent article its applica- 

 tion to the phenomena of electrolysis, and its intrusion into the 

 hypothesis of electrolytic dissociation, together with the quanti- 

 tative developments which have given to the theory a period of 

 new vigour, and have made it one of the foremost subjects of 

 investigation at the present time. 



Reversibility of Hydrate Formation 



Mendeleef and those who followed him most closely appear 

 to have regarded the process of solution as essentially chemical 

 in character. The water dissolved the salt because it combined 

 with it, and this combination involved practically the whole of 

 the water as well as the whole of the salt. Whether at 

 extreme dilutions a mere trace of solute was to be regarded 

 as in combination with the whole bulk of the solvent is not 

 quite clear, but it is at least evident from Mendeleefs paper 

 that a molecule of sulphuric acid was thought to be capable 

 of uniting with the greater part of the first 150 molecules of 

 water which were added to it, so that uncombined water could 

 only be present to any marked extent in solutions containing 

 less than 3*5 per cent, of acid. Similar views were advocated 

 by Pickering {Trans. Chem. Soc. 1890, 57, 129), who suggested 

 that 1800H0O was a reasonable estimate of the complexity of 

 the molecule of water itself, and extended Mendeleefs list of 

 hydrates by the addition to it at one end of the series of the 

 compounds 3H 2 S04 . 2H2O, 3H2SO4 . H 2 0, 6H 2 S0 4 . H 2 0, and 

 36H 2 S0 4 . H 2 0, and at the other end of the compounds 

 H 2 S0 4 . 5o8H 2 0, H 2 S0 4 . 1600HA and H 2 S0 4 . 5i82H 2 ! Inter- 

 mediate compounds with iH 2 0, i'5H 2 0, 2H0O, 5"5H 2 (replacing 

 Mendeleefs H 2 S0 4 . 6H a O), 9HA 13HA 14HA 58HA and 

 i3oH 2 (replacing Mendeleef s H 2 S0 4 . i5oH 2 0) brought the total 

 list of hydrates up to sixteen in this one series of solutions. 



These extreme views as to the complexity of the hydrates 

 existing in aqueous solutions never gained general recognition, 

 but on the contrary were responsible for creating a widespread 

 prejudice against a theory of solutions which was otherwise in 

 many respects so obvious as scarcely to demand a formal proof. 

 This prejudice was extended in an even more marked degree 



