on Supersaturated Saline Solutions, 225 



contact witli tlie sides of the flask, does not undergo m one or many 

 points a notable diminution of surface-tension. 



II. Tliat if we deposit on the surface of a supersaturated saline 

 solution a drop of a liquid of feeble tension, it spreads, and crystal- 

 lization takes place immediately or after a short time. 



III. That while a liquid of feeble tension produces crystallization 

 after a time more or less short, a liquid of considerable contractile 

 force (such as pure water) not acting chemically on the solution, may 

 be brought into contact with it without producing change of state. 



IV. That as a liquid of feeble tension produces crystaUization, so 

 a solid covered more or less with a film of such liquid produces change 

 of state, either at once or after a short time. 



But before any conclusions could be drawn from the results of 

 experiments as to the relation between the surface-tension of liquids 

 and the state of supersaturation in saline solutions, it was necessary 

 to measure the surface-tension of the solutions of Glauber's salt 

 operated on. Accordingly the following data were determined, ^?'<s^, 

 for a solution containing 1 part of salt to 1 of water, and, secondly, 

 for a solution containing 2 parts of salt to 1 of water. The dia- 

 meter of the capillary tube was 1*598 millim.* 



Specific gravity of the solution 1 salt to 1 water at 17° C.= 

 M98. 



The capillary height 1 1 millims. 



The specific gravity of the other solution = 1*289. 



The capillary height 8*7 millims. 



These data give, according to the formula t= ^ • ^ ([n which 



t is the tension, h the height, d the density, and r the radius of the 

 tube), for the superficial tensions of the solutions in question, not 

 a greater value than from 4 to 5*2. 



If the states of supersaturation of saline solutions depend on the 

 maintenance of surface-tension, according to the first proposition, 

 any force or substance that produces a notable diminution of such 

 tension will cause the state of supersaturation to cease. 



Such a force is heat, while such substances as camphor, benzoic 

 acid, &c. have a marked eifect in lowering the superficial tension of 

 water, and in doing so undergo those remarkable gyrations which are 

 so well known. 



And first with respect to heat, applied not so as to affect the 

 whole solution, but locally, so as to raise the temperature at one part 

 or point of the surface, while the other parts remained at the tem- 

 perature of the atmosphere. 



Experiment 1. Four flasks, each about half full of a supersaturated 

 solution of Glauber's salt (2 salt to 1 water), were exposed to a tem- 

 perature of 32° F. for an hour. A red-hot poker was then passed 

 down the neck of each flask, and in two of them the hot metal was 

 brought into contact with the surface of the solution so as to raise a 

 volume of vapour. There was no separation of salt in any one case. 



Experiment 2. A solution containing a considerable mass of the 

 =^ The tube was calibrated by Dr. E. J. Mills, F.C.S. &c. 



Fhil. Mag, S. 4. Vol. 44. No. 292. Sept, 1872. Q 



