CELLS AND MANOMETER ATTACHMENTS. 17 



Occasional experiments with a view to producing a half-porcelain, 

 half-porous cell have been carried out along two lines: first, by so mix- 

 ing the two kinds of clays that for a certain distance from the center, 

 upward and downward, each kind would disappear gradually; second, 

 by mixing some of the glazing material with the clay which was to form 

 the upper half of the cell. The results have been encouraging, though 

 up to the present time not wholly satisfactory. 



THE MANOMETER ATTACHMENTS OF THE CELLS. 



Great difficulty has been experienced in devising suitable arrange- 

 ments for attaching the manometers to the cells. The problem is less 

 simple than it might appear to be at first sight. Three things must be 

 provided for in any workable device for closing the cell: (1) a junction 

 which will not leak at high pressure; (2) means of adjusting, at will, the 

 pressure in the cell (this is especially necessary when manometers of 

 large capacity are used) ; and (3) an arrangement so simple in manipu- 

 lation that the cell can be filled and closed and the proper initial pres- 

 sure established in a fraction of a minute. Several schemes have been 

 employed for joining the cell to the manometer, all of which, with two 

 exceptions, are still in use. Some of the arrangements which worked 

 satisfactorily at moderate temperatures failed utterly at high temper- 

 atures. 



The first crude experiments* were made with cells into which rubber 

 stoppers — carrying manometers — were thrust and fastened in place as 

 well as might be with wire. The highest pressure obtained by such 

 means was only 4.5 atmospheres. The manometers were pushed out of 

 the cells and, owing to the tendency of rubber to flow into regions of less 

 pressure, the stoppers were badly distorted. The earlier experiments, 

 however, were only qualitative. They were made in order to test the 

 membrane rather than with a view to measuring osmotic pressure. 

 Other qualitative experiments were carried out laterf with somewhat 

 improved apparatus, but the earliest successful attempts^ to measure 

 osmotic pressure were made in the apparatus shown in Figure 7. 



The porous cell (A), which is unglazed, is ground out internally to a 

 distance from the open end which is a little over one-third its depth, 

 until the shoulder formed at the bottom of the ground part extends 

 entirely around the cell and is of sufficient width to afford an ample sup- 

 port for the soapstone ring (6) . Afterwards two channels, one of which 

 is designated in the figure by the letter a, are cut into the wall to pre- 

 vent the dislodgment of the cement under pressure. The glass tube 

 (B), which connects the cell with the manometer, is enlarged in two 

 places (c and d) to prevent its displacement, and is contracted at the top 

 to give it a better grip upon the rubber stopper (e). The soapstone 

 ring (6) is accurately fitted to its place in the cell and also to the glass 



*Amer. Chem. Journal, xxvi, 80. \Ibid., xxviii, 1. %Ibid., xxxiv, 1. 



