MEASUREMENT OF OSMOTIC PRESSURE 557 



effects which may persist during many hours or even days after 

 the temperature has again been brought under control. This 

 must necessarily be the case, since the cooling of the cell 

 diminishes the volume of the contents, reduces the internal 

 pressure and permits water to enter the cell, thereby causing 

 a local dilution which may persist during several days. Con- 

 versely, if the cell becomes heated when a condition of 

 equilibrium has been attained, the expansion of the contents 

 will drive water from the cell and concentrate the solution ; if 

 the dense concentrated liquid should sink to the bottom of the 

 cell, much time must be allowed for it to rise again by diffusion 

 and ultimately regain its normal concentration. 



Similar conditions prevail when the cell is first closed. 

 Not only must pressure (approximately equal to the osmotic 

 pressure expected) be applied immediately to prevent water 

 from entering the cell and diluting the contents but this must 

 be done at the right temperature. The whole of the apparatus, 

 solutions, water, etc., must therefore be kept in a thermostat in 

 readiness for setting up. 



The " thermometer-effects " due to fluctuations of temperature 

 were eliminated by using a series of thermostatic devices to 

 control the temperature of large water-baths and air spaces. 

 These were all constructed on one common principle : water or 

 air is passed rapidly (1) over a continuously cooled surface, then 

 (2) over a heated surface which is more efficient but is under the 

 control of a thermostat, (3) thence into or around the space 

 occupied by the apparatus, again over the cooled surface and 

 so on. Figs. 4 and 5, which show the thermostatic devices used 

 in the actual measurements of osmotic pressure, are typical of a 

 dozen such baths used for various purposes. 



Fig. 4 shows the water-bath containing the cells. The 

 cooling surfaces 3, 8, 7, etc., are supplied with water from 

 the hydrant cooled, when necessary, before entering the bath 

 by passing it through a coil immersed in ice. The heating 

 surfaces 9 and 10 contain sockets for four lamps the current 

 through which is controlled by the mercury thermostat at 1. 

 By means of the propeller shown on the left of the figure, 

 water is drawn out of the bath through the pipes 12 and 13, 

 brought back again through the pipe 14 and distributed through 

 the bath ; whilst outside the bath, in the short curved pipes 

 leading from 12 and 13 to 14, auxiliary gas-heating can be 



