METHODS OF COLLOID RESEARCH 113 



natural colloids depends chiefly upon their reaction. Experiments 

 are therefore performed with very faintly acid, very faintly alkaline 

 and neutral reactions. Because many substances are destroyed in 

 alkaline or acid solutions, it is necessary to make appropriate pre- 

 liminary tests. Measurements of the amount contained before and 

 after adsorption determine the character of the particular colloid. 

 In this way L. MICHAELIS investigated a number of ferments (see 

 p. 186). 



In the border land between adsorption and chemical combination 

 belong the studies of staining, which open to the histologist a wide 

 field for the application of colloid-chemical knowledge. 



Internal Friction. 



As has been shown, especially by the investigations of WOLFGANG 

 PAULI on albumin, the internal friction or viscosity serves to give 

 valuable information concerning changes in the condition in colloidal 

 solutions. 



In the case of hydrophile colloids an increase of viscosity usually 

 indicates an hydration. 



The relative internal friction is usually determined by taking that 

 of water at the same temperature as equal to 1. This is usually 

 done by allowing a given amount of fluid to flow from a capillary 

 tube, taking the time with a stop watch. If the rate of flow for water 

 has been previously determined, the relation between the two gives 

 the relative internal friction. 



WILHELM OSTWALD constructed a well-adapted apparatus (described 

 in OSTWALD-LUTHER'S " Textbook and Manual for the Performance 

 of Physico-chemical Measurements," which see for details). The 

 colloid investigator should not work with capillaries that are too fine, 

 because his fluids are usually very viscous. The maintenance of a 

 constant temperature is of especial importance, and therefore it is 

 necessary to employ a transparent thermostat. Furthermore the 

 specific gravity must also be taken. OSTWALD-SPRENGEL'S pyknom- 

 eter may be used. 



In biological investigations it is occasionally necessary to work 

 with very small amounts of fluid. Special apparatus has therefore 

 been devised so that but one or two drops may suffice for a viscosity 

 determination. The apparatus of HIRSCH and BECK, thoroughly 

 described by P. T KORAN YI and A. v. RICHTER, " Physical Chemistry 

 and Medicine II," p. 27 et seq., is frequently used. The apparatus 

 of H. A. DETERMAN is very simple; as seen from Fig. 24, it resembles 

 an hour glass. The capillary has at either end an enlargement, and 



