CHEMISTRY. 359 



onl}^ a few of the qualities of osmotic cells which are unconditionall}' 

 essential to the correct measurement of pressure will serve to impress 

 others with the fact that the labor involved is very great, and the 

 chances of disappointment very many. 



The membrane must consist of an infinite number of little plugs 

 driven tightly into the inner mouths of the cell pores. A membrane 

 of any other structure or location is useless. If it merely covers the 

 inner surface of the cell, it can not resist pressure. If, on the other 

 hand, it is located within the cell-wall at a distance from the inner 

 surface, difficulties due to retarded diffusion occur. The inner sur- 

 face is, then, always bathed by a diluted solution and the pressures 

 observed are neither constant nor at any time the true osmotic pres- 

 sure of the solutions. Inasmuch as the membranes are formed elec- 

 trolytically by the meeting of certain oppositely moving anions and 

 cations, provision must be made such that the meetings can occur only 

 in the right place, i. e., just within the mouths of the pores which open 

 on the inner surface of the cell-wall. This condition is secured by so 

 regulating the size of the pores that, while the large and slowly moving 

 anion is just entering the mouths of the pores from within, the smaller 

 and more rapidly moving cation can pass nearly through the cell-wall. 



The proper regulation of the texture of the cell-wall is a matter of 

 great delicacy and difficulty. If the pores are too large, the membrane 

 is formed too far within the wall; if they are too small, the membrane is 

 deposited on the interior surface of the cell- wall. In either case, the cell 

 is worthless for the measurement of osmotic pressure. We divide into 

 two classes those cells in which the membranes are properly located, 

 designated as ''quick" and ''slow." In cells of the former class, the 

 pores are (within permissible limits) relatively large, and the membranes, 

 therefore, have considerable active area, which accounts for the rapidity 

 with which equihbrium pressures are established. In the second class 

 the pores are relatively small, and the cells are slow in developing pres- 

 sure because the effective area of the membranes is small. 



The required fineness of texture in the cell- wall is secured: (1) by 

 elutriating the clays and using onlj' the finer products; (2) by subject- 

 ing the clay to enormous pressure while forming the cell; and (3) by 

 burning the cells at high temperature. Each process plays its part in 

 determining the eventual size of the cell-pores, and each has been made 

 the subject of elaborate investigation. The baking of the cell is the 

 most critical of all the operations determining the size of the pores, 

 because it is the last one to which the cell is subjected. 



In former years, as stated elsewhere, our cells had been burned at the 

 Chesapeake Pottery. With the cooperation of the officials of that 

 concern, and after much investigation, it was determined under just 

 what conditions, as to location in the kilns, number of repetitions of 

 the baking process, etc., cells of the required texture could be produced. 

 Unfortunately for the progress of our work, the Chesapeake Pottery 



