626 PROCEEDINGS OF THE AMERICAN ACADEMY. 



tions. In particular we have set for ourselves the problem to calculate 

 from these rings the rate of diffusion of the reacting substances, and to 

 estimate the limit of supersaturation attained when the solid phase is uot 

 present, — the so-called " Metastable Limit" of Ostwald. 



Before proceeding to the experiments we shall give certain definitions 

 and explanatory paragraphs from Ostwald's Lehrbuch.* 



" Between a solid substance and its supersaturated solution an equilib- 

 rium is formed, which depends on the nature of the substances, the tem- 

 perature, and the pressure. If we leave out of account the influence of 

 the pressure, for every temperature there is a definite equilibrium. This 

 is evidenced by a definite concentration up to which the solid substance 

 dissolves. ... 



" When the solid phase is not present, the concentration of the dissolved 

 substance is arbitrary. The limit of concentration on one side is zero ; 

 on the other side the limit is a concentration difficult to determine, which, 

 however, is greater than that of the equilibrium with the solid phase. 



" Solutions whose concentrations exceed the concentration of saturation 

 with a possible solid phase are called supersaturated with reference to 

 this solid phase. . . . 



" The simplest way of obtaining a supersaturated solution is to produce 

 by change of temperature from the solid substance and the solvent a solu- 

 tion that is more concentrated than a saturated solution at the temperature 

 of the experiment. In most cases the temperature required to produce 

 supersaturation is higher than the temperature of the experiment, but it 

 can also be lower in cases where the solubility decreases with increase 

 of temperature. 



" A better way to obtain supersaturated solutions, since it is thereby 

 easier to exclude germs of the solid phase, is to employ reactions that 

 produce in the solvent concentrations of the substance in question in 

 appropriate amount. 



"Among supersaturated solutions there are some which under definite 

 conditions can be kept indefinitely, if germs are excluded, without forma- 

 tion of the solid phase. Such solutions are called Metastable. 



" On the other hand, there are some solutions in which, even when 

 germs are excluded, the solid phase appears after a short time. Solu- 

 tions of this type are called Labile. 



" Metastable solutions always show smaller concentration than the 

 labile solutions of the same substances. By increase of concentration, 



* Lehrbuch, II. 2, 780-784. 



