60 REPORT— 1901. 



luaintenauce of the observatories to the end of 1901. AVe have the 

 further pleasure of adding that Mr. Bernard has most generously given 

 a fourth donation of 500^., in addition to the 1,-500/. previously given by 

 him ; and the Meteorological Council have agreed to continue their grant 

 of 250/. to the Low Level Observatory for another year. Provision is 

 thus made for the maintenance of the two observatories to the clo.se 

 of 1902. 



In the meantime the printing of the observations made at the two 

 observatories since 1888 proceeds, and already the first of the three 

 quarto volumes has been printed, and will be issued in the course of 

 next winter. In addition to the observations, this volume will also con- 

 tain several papers and discussion, many of which have been laid before 

 the British Association in our reports from year to year. The publi- 

 cation of these volumes has been undertaken by the Roy.-?! Societies 

 of London and Edinburgh, and the cost is estimated at 1,000/. 



The Clearing of Turbid Solutions, a-iul the Movement of Small Sus- 

 pended Particles hy the Influence of Light. By Professor G. 

 Quincke, of Heidelberg. 



[Ordered by the General Committee to be printed in extenso-l 



By ' turbid solutions ' or ' suspensions ' (triibe Losungen, Triibungen) I 

 mean water in which many small solid or fluid particles are suspended for 

 a long time. The small particles are visible with the microscope. 

 Colloidal solutions with doubtful character will be discussed later. 



Sedimentation, or the formation of flocks, flocking, is observed if small 

 quantities of acid or salt solutions are brought into contact or are mixed 

 with the turbid solution. 



For instance, the sandbank at the mouth of a river is the efiect of the 

 clearing power of the sea water on the particles of clay suspended in the 

 fresh water of the river. 



Turbid solutions of clay, kaoline, silica, gum mastic, are flocked by 

 quantities of acid or salt so small that the increase of weight by the 

 clearing substance cannot explain the augmented velocity, or the flocking 

 of the falling particles, or the sedimentation of the turbid solution. 



Franz Schulze ' and Schloesing '^ found xj)W(So *o itfoWo of calcium or 

 magnesium salts sufficient to clear suspensions of clay. Bodlander ^ has 

 measured the clearing or coagulating power of different salts for suspen- 

 sions of kaoline ; Hardy * for suspension of gum mastic ; Spring ^ for sus- 

 pensions of gum mastic, kaoline, silica. Bodlander found suspensions of 

 kaoline flocked if the quantity of the added salt is greater than a distinct, 

 .very small quantity, the ' Schwellenwerth ' of the clearing substance. 

 Electrolytes promote, insulators retard, the clearing of the suspensions, 

 (Barus,^ Bodlander). The clearing power of a salt depends on the valance 

 of the salt and the kation of the electrolyte (Hardy, Spring). 



According to Hardy, the particles of gum mastic, or heat-modified 



' Franz Schulze, Poggendorff' s Annalen, 1866, vol. csxix. p. .366. 

 2 Ch Schloesing, Coinpt. Itend., 1870, vol. Isx. p. 1345. 

 ' O. Bodlander, Gott. Nachr., 1893, p. 267. 



* W. B. Hardy, Proc. Rcry. Soc, 1900, vol. Ixvi. pp. 111-119. 



* W. Spring, Rec. Trar. CJiim. des Pays-Bas, 1900, vol. x. (2 ser. 4), no. 3, pp. 

 222, 294. 



« C. Earns, Pliys. Beihl, 1888, vol. xii. p. 563. 



