Reaction before Complete Equilibrium. 63 



with variation of temperature, and partly to the variation 

 with temperature in the form of the crystals, and therefore 

 of the surface of contact of the solid with the liquid. Since 

 all these unavoidable complications exist and prevent our 

 arriving at the true nature of the phenomenon, and since the 

 reaction cannot be investigated at all for several degrees near 

 the melting-point, — -just in the region which is of the greatest 

 theoretical interest, — this method of investigation had to be 

 given up and replaced by another free from all the above 

 sources of error (see § III.). One point has still to be mentioned 

 here : we must always make a distinction betiveen the region 

 of Crystallization and that of Solidification. 



If the latent heat of melting of 1 kilogram of the solvent 

 or of the solution be X, the specific heat of the liquid =0"', 

 of the solid = C lv , then the maximum number of degrees which 



i t • i • ^-(1 — n r) a i 



the liquid can rise is j-^-, • ,,.,,, : . As lono- as 



1 n T + lv (l — n T ) ° 



T » _T< C"\ + j"(l-n r ) at the time T > 

 the system transforms itself into one in which the solid, liquid, 

 and vapour are in equilibrium. When, on the contrary, 



T — T became > -^ . ,. ,\ -, the svstem becomes one in 



— Kj n T -\-L™{L — n T ) J 



which the solid and vapour only are in equilibrium. In the 

 first case we have the phenomenon of crystallization, the 

 velocity of reaction is directly proportional to T r — T, and the 

 point of equilibrium is always the same T . In the second 

 we have the phenomenon of solidification, and since the solid 

 and vapour can be in equilibrium at all temperatures, the 

 point of equilibrium (T ) is by successive undercoolings con- 

 tinuously lowered, being always at a distance t-^- , iv T . 



\j tl T ~f-\j ^_L n T ) 



from the temperature of the liquid at the surface of contact, 

 while the reaction is taking place ; in other words, the velocity 

 of solidification is independent of the amount of undercooling. 

 Moreover, since the value \ decreases with the fall of tem- 

 perature, we have to expect, assuming the values of C"', C iv to 

 remain the same, that the velocity of reaction may even 

 become smaller with the decrease of temperature. 



2. The Velocity of Reaction before equilibrium between the liquid 



solution and the bolid solution takes place. {SoHdiji<-ation of 

 Phenol and Metacresol.) 

 It was found that phenol and metacresol form solid solu- 

 ions, i. e. the metacresol is partly dissolved in the liquid and 



