EXPERIMENTS OF PFEFFER. 



227 



or clay cell, like those which are used in the Bunsen battery, 

 connected by means of a glass collar with a suitable manometer. 

 Within the clay cell a precipitation film is formed ; 1 the cell is 



1 The following account of details essential to success in these experiments 

 of Prof. Pfeffer has been prepared by one of his students, Dr. W. P. Wilson. 



The principal portion of the apparatus is a porous porcelain cell, z, 46 mm. 

 high and 16 mm. in diameter, with walls l mm. in thickness. This cell is 

 cemented on to a piece of glass tubing, v. A second piece of tubing, t, with 

 lateral tube, is cemented into the first piece. The lateral opening is for the 

 manometer m, the one at g is for the convenience of filling and sealing th 

 cell. 



One of the two fluids used in forming the 

 membrane for experimentation is allowed 

 to penetrate the porous cell from without. 

 When this has thoroughly taken place, the 

 second fluid is poured into the interior. The 

 contact of the two fluids takes place, there- 

 fore, on the inner surface of the porous cell, 

 and here the precipitate is formed which is 

 termed the pellicle-membrane or precipita- 

 tion-membrane. Substances which by their 

 mutual contact give rise to such precipitation- 

 membranes are termed membranogenic. It 

 will readily be seen that during any internal 

 pressure the porous porcelain cell acts as a 

 support for the membrane. If the exterior 

 solution is copper-sulphate, the interior solu- 

 tion potassic ferrocyanide, then the precipi- 

 tated membrane will be cupric ferrocyanide. 

 After the membrane has been formed, then 

 any solution not chemically incompatible 

 with it may be employed in the cell ; namely, 

 syrup from cane-sugar, a solution of saltpetre, 

 or a still stronger solution of potassic ferro- 

 cyanide than was used in the preparation of 

 the cell. 



As the successful working of the apparatus 



depends upon the exact carrying out of quite a number of minor details, the 

 following description of the methods of putting the parts together may be 

 found useful : 



In order to insure absolute freedom from any foreign substance, the porcelain 

 cell must be successively washed in dilute solutions of potassic hydrate and 

 hydrochloric acid, and then thoroughly dried. Warm a piece of sealing-wax 

 in the spirit-lamp and draw it to a point. Slowly heat the open end of the 

 cell in the alcoholic flame. When hot enough to very readily melt the wax, 

 apply the point; and while the cell is continually rotating, cover evenly a space 

 to the depth of 15 mm. with wax in the interior. It should be about 2 mm. 

 in thickness. Pick up the short piece of tubing, v, which has been previously 

 waxed on one end, and rotate it over the flame. When both porcelain cell and 

 glass tube are as warm as they can be made and yet the wax kept smooth 



