PHYSICAL CONSTANTS OF SATUEATED SOLUTIONS. 195 



obtained, the numbers being " meaned " as before. The process was then continued 

 for the next higher temperature, and so on. The object of adding boiling water 

 is to make sure that the solution is unsaturated, for if cold water were added the 

 temperature of the solution would fall, and if the rate of attaining saturation be 

 greater than the rate at which the solution comes back to the constant temperature, 

 you get a solution supersaturated with respect to that temperature. 



It will therefore be seen that the method adopted resolves itself into this : at any 

 given temperature, two observations of density and solubility are taken; one is 

 obtained by stirring a supersaturated solution in contact with the solid salt, the other 

 by stirring an unsaturated solution in contact with an excess of salt and the true 

 density or solubility is considered to be the mean of the two observations. 



In the earlier part of this work it was found that, in many cases, a very long time 

 elapsed before the densities obtained, when starting with an unsaturated solution, 

 approached sufficiently closely to that derived by starting with a supersaturated one 

 this was partially remedied by increasing the speed of stirring from 2 to 20 revolutions 

 per second but even then there was generally a difference in the two densities of 

 some few units in the 4th decimal place. The cause of this discrepancy was 

 eventually traced to the fact that a considerable length of time was also required for 

 the point of saturation to be attained by a supersaturated solution, even when stirred 

 in contact with its salt. It was owing to this that some 300 density and solubility 

 determinations had to be discarded for preliminary observations had shown that 

 concordant results could be obtained by merely covering the top of the beaker with a 

 glass perforated for the stirrer to pass through, and removing the plate while the 

 pyknometer was being filled. At the higher temperatures the removal of the plate 

 caused a fall in the temperature of the solution and a consequent supersaturation. 

 This, however, was not suspected (because when the results were plotted the curve 

 was regular) until I was dealing with very soluble salts, which, on the removal of the 

 plate, tended to form crusts of salt on the surface of the solution. The difficulty 

 was overcome by closing the beaker by an india-rubber stopper, which was perforated 

 for the stirrer, the thermometer, and the pyknometer. The latter was closed at the 

 upper end by a tap attached by rubber tubing. The tap served two purposes : it was 

 kept closed on immersing the pyknometer, so that no liquid could enter during the 

 time that the pyknometer was attaining the temperature of the solution, and it was 

 closed after filling the pyknometer, so that no liquid could flow back during the 

 withdrawal of the rubber stopper. 



As an extreme example of the necessity of giving an unsaturated solution plenty 

 of time to attain saturation, and also as showing the importance of having a sufficiency 

 of salt in contact with the solution, I extract the following numbers from my note- 

 book. An unsaturated solution of thallium alum, together with a quantity of the 

 salt, was placed in the beaker, which was at the constant temperature of 61'0 C. 

 This was stirred for 12 hours at the rate of 10 to 20 revolutions per second; at the 



2 c 2 



