ON THE CLEARING OF TURBID SOLUTIONS. Gl 



proteid move in a contrary direction to an electric current. In presence 

 of a minute amount of barium chloride or free acid the particles of 

 gum mastic, or heat-modified proteid, move with the electric current. At 

 the isoelectric point, for a distinct small quantity of barium chloride or 

 acid, the electric movement vanishes and coagulation or precipitation 

 occurs. An explanation of the clearing power of the acids or salts is not 

 given. 



In the coagulated solutions I found flocks adhering to the walls of the 

 glass vessels and many air bubbles distributed among the flocks. Both 

 phenomena prove that on the surface of the flocks at least, a short time 

 after formation of the flocks, an oily viscous fluid exists. At the surface 

 of separation of this oily fluid and the surrounding aqueous fluid, a 

 surface tension acts and air bubbles are separated, as at the limit of two 

 heterogeneous fluids. Probably changes of the surface tension of the 

 boundary of oily and aqueous fluid and the periodical spreading of hetero- 

 geneous liquid will excite vortices and unite the small suspended particles 

 and form the flocks. The surface forces are the same as the forces which 

 form foam-cells by the contact of alkaline oleates with water, which I 

 demonstrated at the meeting of the British Association at Oxford, 1894. 

 The flocking influence of quantities of clearing matter so very small is 

 now intelligible. 



I shall prove that this explanation is the right one. 



Alcoholic solution of gum mastic gives in a large mass of water many 

 unseen threads and foam- walls, in which are distributed a great many 

 small visible spheres. If copper sulphate is added to the water with the 

 mastic foam the foam-walls move against the copper sulphate, become 

 clearer, and are dissolved. The spheres and the foam-walls prove tlie 

 formation of an oily viscous fluid by the action of water and gum mastic, 

 which I will call mastic hydrate, and which possesses a surface tension at 

 the surface of separation from water. The copper sulphate is soluble in 

 water and in mastic hydrate, has the surface tension zero at the boundary 

 with water, and in the boundary with mastic hydrate, and must be spread 

 out on the common surface of mastic hydi-ate and surrounding water. 

 The spreading excites vortices and draws the surrounding matter towards 

 the spreading centre ; the surrounding fluid is stirred up, a new portion 

 of copper sulphate is brought into contact with the mastic surface, spreads 

 out, and so, in short periods, the spreading of the added salt and the 

 formation of vortices are repeated, and the mastic particles are attracted 

 by the copper solution. 



The solution of copper sulphate, which is placed by means of a long 

 thin funnel under a turbid solution of mastic in a test tube, will difi'use 

 in the mastic solution, spread out on the surface of the suspended par- 

 ticles, excite vortices, and draw the mastic particles together or against 

 the walls of the test tube, where they will adhere. The connected viscous 

 matter will flow together and form drops, bubbles, or coherent foam- 

 cells, flocks. On the surface of the mastic hydrate, as in all newly formed 

 boundaries of two hetei'ogeneous fluids, the absorbed air is separated in 

 small bubbles. One part of the flocks will rise with the adhering air, the 

 other part with the larger flocks will sink to the surface of the salt 

 solution. 



The spreading or vortices of sufficient energy and the connection or 

 flocking of the suspended particles demand a certain concentration of the 

 copper sulphate, corresponding to the ' Schwellenwerth ' of Bodlander. 



Solutions of NaCl, HCl, K2Cr207, FeCl^, spread out on the surface of 



