512 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



On the other hand, at 35° Cottrell used only two hours for saturation, 

 while we used about seven times as much time. Hence the difference 

 between his result, 67.87, and ours, 68.22, may be explained once more 

 by a difference in the time of mixing. 



Until recently the cause of this difference would have been ascribed to 

 a possible incomplete saturation in Cottrell's case ; and the higher figures, 

 other things being equal, would have been accepted as the more accurate. 



While this may be true, the recent work of Ostwald* on the solubility of 

 powders and the surface tension of solids has thrown new light on this 

 matter, and it seems quite possible that neither series of results may be 

 perfectly definite. Fine powders have a greater solution-tension than 

 coarse ones, for the same reason that small drops have a greater vapor 

 tension than large ones. 



It has often been stated that very long agitation is necessary to secure 

 saturation. t In view of Ostwald's newer work it seems quite possible 

 that continued active agitation introduces an uncertainty even greater 

 than the one which it avoids. The crystals of salt in the ever-moving 

 tube act as mutual millstones, and gradually wear off one another's 



* Ostwald, Z. phys. ch., 34, 495 (1900). 



t For example, see Ostwald's Pliysicochem. Measurements, Walker (Macmillan, 

 1894), p. 176. 



