METHODS OF COLLOID RESEARCH 



119 



H. BECHHOLD'S "Bell apparatus." 1 The colloid solution to be tested 

 is placed in the glass vessels AA (see Fig. 27) which are connected 

 by a tube. The vessels are closed below by the membranes MM 

 (best for the purpose is fish bladder or the like). The tube R 

 allows for the expansion of 

 the fluid caused by the heat 

 of the electric current. The 

 bell apparatus is placed in two 

 separate glass vessels (crystal- 

 lizing dishes) GG] the mem- 

 branes are immersed in the FlG 2 7. Bell apparatus. (H. Bechhold.) 

 water, into which also dip 



the electrodes EE. The advantages of the apparatus are: the 

 great surface of colloid solution; the products transferred do not 

 come in contact with the electrodes and each may be conven- 

 iently and separately collected and ex- 

 amined; the current must pass through 

 the entire colloidal solution; the ap- 

 paratus may be easily sterilized and the 

 free surfaces may have toluol layered 

 over them. The apparatus does not get 

 out of order very easily. L. MICHAELIS 

 has avoided the change in reaction due 

 to electrolysis at the electrodes by using 

 nonpolarizable electrodes. Fig. 28 amply 

 explains the apparatus. The electrodes, 

 e.g., zinc or silver wire, are dipped into 

 the vessels 1 and 5, which are filled 

 Fia. 28. Migration apparatus with zinc su l p h a te and NaCl solution 

 with non-polarizable dec- rpsnprtivelv 

 trodes. (L. Michaelis.) respectively. 



Migration experiments are usually very 



difficult to perform. Since the nature of the charge may also be 

 determined by adsorption, by employing positive and negative 

 adsorbents, this latter method is preferable, because it is simpler. 



&. 



Optical Methods. 



There is a certain relation between the cloudiness (Tyndall effect) 

 of a fluid and its content in suspended particles or colloid. On this 

 account various authors (KAMERLINGH ONNES and KEESOM, MECK- 

 LENBURG, WILKE and HANDOVSKY) have constructed instruments to 



1 To be had from the Vereinigten Fabriken fur Laboratoriumsbedarf , Berlin N. 

 Scharnhorst Str. 



