1C OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



in such cases, the ferrocyanogen ions do not get far enough into the pores 

 before meeting those of copper coming from the opposite direction — in 

 other words, that the membrane is formed at or without rather than 

 within the mouths of the pores. 



After baking the cells and before glazing them, they are mounted on 

 the lathe and ground under the shoulder with a high-speed carborundum 

 wheel, to fit the brass rings with which the manometers are fastened 

 in their places. The necks are also ground to the exact taper of the 

 cones upon the ends of the manometers. 



The finding of a suitable glaze for the upper half of the cells was a 

 matter of considerable difficulty. As might have been expected, the 

 expansion coefficient of products made as these cells are is very different 

 from that of any of the potters' wares. Hence none of the glazes which 

 are used by the potters would meet the requirements of the situation. 

 All such glazes were found to "craze" badly upon the biscuit. An 

 attempt "was made to glaze with feldspar, but with poor success. A 

 wholly suitable glazing material was finally obtained by adding silica 

 and feldspar to one of the glazes which are used by the potters upon the 

 better grades of their white tableware. The earlier experiments in 

 glazing were carried out in a Seger gas kiln, but at the present time the 

 glazing, as well as the baking of the cells, is done at the potteries. 



There is one objection to glazing the cells to which attention should 

 be called. They are glazed, inside and outside, from the middle 

 upward, leaving the lower half of the cells porous. The whole interior 

 of the cell is therefore protected at all times, either by the glaze or 

 the membrane, so that no material in solution can diffuse into the wall 

 from the inside. On the outside, the case is different. There it is quite 

 possible for the dissolved substances to diffuse upward and accumulate 

 between the inner and outer glazed surfaces. If these were allowed to 

 remain and should afterwards diffuse downward and distribute them- 

 selves about the membrane, the pressure measured would not be that of 

 the solution within the cell, but rather the difference between the pres- 

 sures of the solutions on the opposite sides of the membrane. It is not 

 believed that the results to be reported in later chapters have been at 

 all vitiated by this possible source of error; because it has always been 

 necessary, in order to maintain unimpaired the colloidal state of the 

 membrane, to soak the cell for considerable intervals in pure water 

 between any two successive experiments. Nevertheless, it seemed de- 

 sirable to produce a cell, the upper half of which has the non-permeable 

 character of porcelain, while the lower half remains porous. The diffi- 

 culty is, of course, to prepare clay mixtures for the two parts of the cell 

 which shall maintain identical expansion coefficients throughout the 

 whole of the baking and cooling periods — at least at all points of union 

 between them. Otherwise cracks or a condition of weakness must 

 develop at the junction of the two clays. 



