CELLS AND MANOMETER ATTACHMENTS. 15 



potteries, we could neither control nor know with certainty the tem- 

 perature of any part of the kilns, but the places in them where the best 

 results are most frequently obtained were easily found, and since then 

 all of the cells have been burned at the potteries. 



It has been stated elsewhere that, in the endeavor to produce the cor- 

 rect texture of cell wall, we made a study of thin sections, both of the 

 potters' cells and of our own. In the course of this work, a considerable 

 number of photographs were accumulated, 6 of which are here repro- 

 duced. Three of them (Plate 1, a, c, and e) are from potters' cells, and 

 three (b, d, and /) are from the first cells made by us which proved them- 

 selves well suited to the measurement of osmotic pressure. It will be 

 noted that the texture of the cells made in the laboratory is incom- 

 parably finer than that of the potters' products. But we were con- 

 vinced, after nearly five years of laborious investigation, that just this 

 excessive fineness of texture is absolutely indispensable to the correct 

 measurement of osmotic pressure. It is necessary, in the first place, in 

 order that the membrane may be deposited exclusively upon the inner 

 surface of the cell wall. It is not meant by this statement that no part 

 of the membrane is to be found within the pores. On the contrary, all 

 good membranes are found, on microscopic examination, to be firmly 

 rooted in the mouths of the pores which open behind it. It is this feat- 

 ure, in fact, which makes membranes produced by the electrolytic 

 method so much superior to those which were made by the older pro- 

 cess. Fineness of texture is also necessary in order to give the mem- 

 brane a backing which will enable it to withstand pressure. If it is 

 more open than that shown in Plate 1, b, d, and /, the membrane is 

 deposited, at least partially, within the cell wall, and it breaks under 

 moderate pressure. 



It is desirable to explain the numerous black specks seen in Plate 1, 

 b, d, and /. They are particles of the emery used in grinding the sec- 

 tions, and no part of what the photographs are intended to show. 



The exact extent to which the sections here represented were magni- 

 fied can not now be stated, the original records having been mislaid or 

 lost, but it is believed to have been 125 diameters. 



The question naturally arises, whether it is possible to make the text- 

 ure of a cell wall too close, provided, of course, it still remains porous to 

 some extent. The effective area of a membrane is equal to the aggre- 

 gate area of the pore-openings upon the interior surface of the cell wall, 

 and it has been found quite possible, by hard burning, so to diminish this 

 area of membrane as to make the passage of solvent into or out of the 

 cell intolerably slow. Some evidence has also been gathered to show that 

 the reduction in the size of the pores may be carried to such an extent 

 that the membrane no longer roots itself firmly into them. This is the 

 explanation given to the formation of the detachable membranes which 

 are sometimes deposited in very hard-burned cells. It is imagined that, 



