18 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



tube (B), the end of the latter having been ground to a perfectly cir- 

 cular form. The lower end of the glass tube is beveled inward to pre- 

 vent the lodgment of air. The purposes of the brass parts (g, h, and o) 

 are obvious without explanation. The tube (B) is set in the brass 

 piece (o) and in the cell (A) with litharge-glycerine cement. But 

 before proceeding to the latter operation, the glass tube, with the soap- 

 stone ring in place, is inverted, and any space which is left between 

 them is filled with molten shellac. The tube and ring are then heated 

 in an air bath until the shellac remains solid at 100°. The cement 

 employed to fix the tube and the ring (B and b) in their places in the 

 cell (A), and also the shellac used to join b to B, must be effectually 

 protected from any contact with the solution in the cell or the water 

 outside of it. For this purpose, the lower end of the glass tube, the 

 soapstone ring, and the whole of the ground surface within the cell are 

 repeatedly painted with a dilute solution of rubber. When a covering 

 of sufficient thickness has been obtained, the soapstone ring — which is 

 now firmly attached to the glass tube — is crowded into its place on the 

 "shoulder." The operation is liable to lacerate more or less the rubber 

 covering of the cell wall. To repair any damage of this kind, and also 

 to insure a tight joint between the clay wall and the soapstone ring, the 

 whole cavity above the latter is again painted with the rubber solution. 

 The apparatus is then placed in an air-bath and maintained at 100° 

 until the rubber becomes quite hard but not brittle. Finally the space 

 between the glass tube and the cell wall is filled with the usual mixture 

 of litharge and glycerine. The lower end of the manometer is enlarged 

 (j) to prevent its being pushed upward through the stopper (k). The 

 purposes of the cork (I) and of the bottle (m) do not require explanation. 

 A special instrument, which came to be known as the "fang," is 

 required both to close and to open the cell. It is shown in Figure 8. It 

 consists of a round, slender, and tapered piece of steel, one end of which 

 has been furrowed out upon one side and bent into the curved form seen 

 in the figure. It was usually made from a small round file from which 

 the temper had been drawn. The "fang" is inserted between the 

 rubber and the glass tube at e, to permit the escape, through the furrow, 

 of the excess of liquid when the cell is closed, and again to provide for 

 the entrance of air when the cell is opened. It is likewise of great assist- 

 ance, when manipulated as a lever, in introducing and removing the 

 stopper through the narrow mouth of the tube. The stopper from e 

 upward is tightly wound with shoemakers' waxed thread to prevent the 



Fig. 8. — The "fang" for the introduction and removal of the rubber stoppers (A;, Fig. 7). 



