20 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



rubber from oozing out of the glass tube. The initial pressure in the 

 cell is adjusted by means of the nut (h) and the collar (g). 



The arrangement described above was employed for the measurement 

 of osmotic pressure from 1905, when the first good cells were obtained, 

 until 1908, when, for reasons which will be stated, but not fully discussed 

 until later, it was abandoned for the apparatus which is shown in 

 Figure 9. 



The principal objections to the first apparatus employed for quanti- 

 tative purposes (stated in the order of their importance) were: 



1. The length of time required to close and open the cell. During 

 both periods, the contents of the cell, being necessarily under less than 

 maximum pressure, became diluted by the water which entered through 

 the membrane. 



2. The difficulty of the manipulation required properly to introduce 

 and remove the rubber stopper without injury to the manometer. 



3. The frequent bursting of the glass tube (B), which was usually 

 attended by the total loss of the cell (A) ; since, as a rule, the membrane 

 was ruined by the measures taken to replace a broken tube. 



The apparatus represented in Figure 9* is a decided improvement on 

 that shown in Figure 7. In it the difficulties enumerated above are 

 obviated, though, as will be seen later, it has certain defects of its own. 

 The function of the brass collar (1) and of the brass nut (2) will be 

 readily understood without explanation. The form of these pieces has 

 varied but little from the beginning. The lead ring (3) separates the 

 shoulder of the cell from the flange of the brass collar and serves to pro- 

 tect the glaze upon the former. A ring of softer material, e. g., leather, 

 can not be used for the purpose, since any upward movement of the 

 collar, due to diminishing thickness of the ring under pressure, leads to 

 an increase in the capacity of the cell and a dilution of the solution. In 

 other words, the ring (3) must be of fairly rigid material. The brass 

 cone (4) has two holes passing entirely through it, one for the mano- 

 meter tube (5) and the other for the hollow needle (6), both of which 

 (the manometer and the needle) are securely fastened in the cone by 

 some fusible metal (Wood's, Rose's, or Babbit's). The holes through 

 the cone are bored slightly larger than the tubes which are to occupy 

 them, in order that the molten metal may flow down and completely fill 

 the space between the latter and the walls of the former. In this way 

 the tubes are more firmly fixed in their places and all danger of leakage 

 upward through the cone is avoided. The hollow tube (6)— the 

 needle — is nickel-plated and is brazed into the brass piece (8), which is 

 bored out and threaded internally at the upper end to fit the closing 

 plug (9). The upper end of 8 and the lower end of the larger portion of 

 9 are made concave in form, and between them is placed the packing 

 (10). The concave form of these two surfaces is essential, since it pre- 



*Amcr. Chem. Journal, xl, 266; xlv, 91. 



