162 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



when its temperature falls from a higher level to that of the bath. Hence 

 it is always intended, when working at high temperatures, to have the solu- 

 tion a little too hot when the cell goes into its final bath. It is not pos- 

 sible, however, to regulate the temperature conditions so perfectly that, 

 after filling a cell and introducing it into the bath, the contraction of the 

 solution will exactly balance the expansion of the mercury in the manom- 

 eter. For that reason the cells are often placed in a so-called "prelimi- 

 nary bath" winch is more accessible and less elaborate than that in which 

 the measurements of pressure are made; and they are there observed while 

 coming to temperature. If the observed pressures are considerably 

 above the approximately known osmotic pressures, small portions of the 

 solutions are allowed to escape from the cells. If, on the other hand, they 

 are much below the true osmotic pressures for the given temperature, an 

 additional mechanical pressure is brought upon the contents of the cells. 

 When the temperature of the cells and their contents has finally reached 

 that of the bath, the pressures should be very nearly equal to the true 

 osmotic pressures of the solutions ; since, otherwise, the inclosed solutions 

 must suffer some concentration or dilution. The supplementary process 

 of pressure-adjustment, described above, can not be dispensed with in 

 high-temperature work. At moderate and low temperatures, sufficiently 

 close adjustments of pressure can usually be secured at the time of closing 

 the cells; that is, the probable changes in the volumes of the solutions 

 and of the mercury in the manometers can be more accurately estimated. 

 Nevertheless, even at low and moderate temperatures, the cells are care- 

 fully watched until it is certain that no further adjustments of pressure 

 will be necessary in order to prevent a sensible change in the concentra- 

 tion of the solutions. The pressures to which the cells are adjusted before 

 placing them in the final bath, or leaving them to come undisturbed to 

 equilibrium, are known as "initial" pressures. They are, of course, only 

 temporary values. 



It has been proved by a large number of experiments that it is imma- 

 terial from which direction the final equilibrium pressure is approached, 

 i. e., whether from a higher or lower initial pressure. It is only necessary 

 that the interval between the initial and final pressures shall not be sufficient 

 to produce — through change in the volume of the cell contents — a sensible 

 concentration or dilution of the inclosed solution. In some series of 

 measurements, it has been customary to so adjust the initial pressures in 

 duplicate determinations that the equilibrium pressure was approached 

 in one instance from above and in the other from below. 



The importance of demonstrating that a solution has maintained its 

 original concentration throughout a measurement of pressure can not be 

 over-emphasized ; accordingly, whenever a cell has been filled and closed, a 

 part of the solution has been reserved for comparison, with respect to 

 concentration, with the solution which was removed from the cell at the 

 close of the experiment. In all the measurements recorded in the present 



