RICHARDS AND FRAPRIE. — MANGANOUS SULPHATE. 513 



corners, with the production of fine powder. This fine powder must 

 continually dissolve, because it is more soluble than the larger aggre- 

 gations. At first it will simply hasten the speed of attaining satu- 

 ration ; but later, when saturation with respect to the larger particles 

 has been attained, the fine powder will tend to produce a solution super- 

 saturated with respect to those larger particles. The experience of 

 Cottrell and others seems to indicate that supersaturation is harder to 

 obviate than inadequate saturation. Cottrell, for example, found that 

 MnS0 4 .H 2 attained a concentration of 57.41 grams in 100 grams of 

 water after two hours of agitation when the salt was added to pure 

 water, while after twice as long a time the supersaturation in another 

 tube had only been reduced from 72.5 to 62.3 grams per hundred. The 

 solubility found after 27 hours, when both methods gave the same result, 

 was 58.32, or 0.91 grams more than the first and 4. grams less than the 

 second figure. 



The outcome of the matter seems to be that constant results upon 

 solubility are usually obtained only when the rate of production of the 

 fine powder exactly balances the rate at which the supersaturation is 

 relieved, and that constant results thus reached represent only a com- 

 promise. With cautious agitation, it is possible that a solubility very 

 near that of flat crystal surfaces might be obtained ; on the other hand, 

 very active agitation, such as we used, must tend to increase this solu- 

 bility almost to that corresponding to the fine powder which we always 

 observed in our tubes. 



Everyone will agree with Ostwald in deciding that the solution tension 

 of flat surfaces, rather than that of sharply curved surfaces, is the quan- 

 tity which should be determined, if possible. This result would be best 

 obtained by keeping the solid as free as possible from agitation, and 

 driving a constant current of the saturating solution over these resting 

 crystals. A dissolving apparatus which has recently been described, if 

 assisted by a small turbine and suitably immersed in a thermostat, would 

 perhaps be the safest apparatus, although complete saturation would re- 

 quire much time.* 



It is evident, as Ostwald points out, that most published determina- 

 tions of solubility, those of Cottrell and our own included, are subject 

 to a small uncertainty, but it is also evident that the work of Cottrell 

 is by far the most complete work upon the solubility of manganous sul- 

 phate which has been published, and that it wholly overthrows the 

 erroneous results of Linebarger. 



* Richards, Am. Chem. Journal, 20, 189 (1898). 

 vol. xxxvi. — 33 



