MEASUREMENT OF OSMOTIC PRESSURE 553 



(3) Unequal porosity even in the same cell, which caused the 

 membrane to wander towards the outer wall at every 

 locality of coarse texture. 

 The problem of making cells in which all the essential qualities 

 should be combined was finally solved by avoiding altogether 

 the use of ground feldspar as a binding material and selecting 

 as raw materials two natural! clays, one deficient in binding 

 material the other over-rich in that constituent ; these could 

 be mixed very intimately and never failed to give products 

 which were perfect in respect of uniform porosity. The carefully 

 prepared mixture was packed into a cylindrical steel mould and 

 subjected during fourteen to sixteen hours to a total pressure of 

 about 200 tons. From these cylinders cells were turned out on 

 the lathe, both the chuck and the cutting tools being of special 

 design ; the difficulty of this operation is shown by the fact that 

 at first 90 per cent, of the cells cracked in the kiln, a proportion 

 that has now been reduced by careful working to about 10 per 

 cent. After baking at about i30o°C. the pots were ground to 

 take the metal fittings and then glazed inside and out, from 

 the middle upwards, with a special glaze prepared by adding 

 silica and feldspar to one of those used by potters for the better 

 grades of white tableware. 



Fig. 2 shows the complete cell as fitted up for use at the 

 present time. The cell and the manometer are clamped to- 

 gether by means of a brass collar (1) and a brass nut (2), the 

 washer (3) being made of lead. The main brass cone (4) is pierced 

 with two holes for the manometer tube (5) and for the hollow 

 needle (6), which are both secured by means of Wood's metal at (7) 

 and later by a cone of the same metal at (1 1). The joint between 

 the metal fittings and the pot is made by means of a rubber tube 

 (i2)|wound tightly at the upper and lower ends with twisted 

 shoemakers' thread (13, 14). The hollow needle (6) is nickel- 

 plated and brazed into a brass piece (8), which is bored and 

 threaded to fit the closing-plug (9) ; the grease-filled leather 

 packing at (10) makes a tight joint when screwed down. 



2. The Manometers. — The form of manometer used in all the 

 later experiments on the influence of temperature on osmotic 

 pressure is shown in fig. 3. The bore is very small, from 

 C45 to 072 mm. The advantages of narrow tubes are 



(a) that the short mercury columns at the top of the 

 capillary are less liable to be displaced by tapping ; 



