18 GENERAL AND PHYSICO-CHEMICAL. 



closer must the filter be Extensive experiments on the filtering of 

 colloids have been carried out by BECHHOLD. 1 He used paper niters 

 which were impregnated with collodion dissolved in glacial acetic acid. 

 According to the concentration of the collodion solution filters of dif- 

 ferent porosity were obtained. The colloid solutions were pressed 

 through the filter by a pressure up to five atmospheres. It was shown 

 that all colloid solutions contained particles of various sizes. Never- 

 theless for every solution a filter could be prepared whose pores were 

 small enough to retain all the particles. In this manner BECHHOLD was 

 able to classify the colloids in a series according to the size of the smallest 

 particles. He found that in general the inorganic colloids (Prussian 

 blue, platinum, iron oxide, gold, silver) form larger particles than the 

 organic colloids (gelatin, haemoglobin, seralbumin, proteoses, dextrin). 

 Still it must be remarked that according to ZsiGMONDY 2 the size of the 

 particles of the same colloid are larger in one preparation than in another 

 and that the size can change on keeping. 



On filtering proteose solutions through filters of unequal thickness 

 BECHHOLD was able to show that the larger the particles of the proteoses, 

 the easier are they precipitable by ammonium sulphate. 



Diffusion. We have already seen that the osmotic pressure of a 

 colloid solution is very small and also that the osmotic pressure of a solu- 

 tion is the cause for the diffusion of the particles, therefore it is evident 

 that the diffusion ability of colloids can only be very slight. This is 

 not only true for the free diffusion but also for the diffusion through a 

 membrane. Both of these was first studied by GRAHAM. The first was 

 found very slight but measurable in several cases while the fact that 

 the colloids did not diffuse through membranes (npn-dialy sable) was 

 given as the most constant difference between colloids and crystalloids. 

 Nevertheless, there does not exist any sharp boundary and dialysis depends 

 principally upon the size of the particles as well as upon the character of 

 the membrane. 



Internal Friction. By the internal friction of a fluid we mean the 

 force which resists the displacement of the particles of the fluid among 

 one another. The internal friction is therefore an expression for the 

 great thickness or viscosity of the fluid. 



For physiological purposes the internal friction is determined by 

 measuring the time which a given volume of the fluid requires to flow 

 through a capillary tube under a pressure of its own weight. 



It is generally accepted that the internal friction of suspension col- 



1 Zeitschr. f. physik. Chem., 60, 257 (1907). 



2 Zur Erkenntnis d. Koll., (1905), 104 as well as Zeitschr. f. Elektrochem., 12, 

 631 (1906). 



