42 PHYSICAL CHEMISTRY IN BIOLOGY. 



ular movement. SVEDBERG also experimentally proved the previously proposed 

 simple law that the distance traveled in a certain time is in inverse proportion to 

 the viscosity of the means of dispersion. Brownian molecular movement is con- 

 sidered by some as a manifestation of a general molecular movement of matter. 



Electrical Transportation of Suspended Particles. A not too 



weak electric current has the power of causing motion in small quantities 

 of fluid enclosed in a capillary tube or in a porous diaphragm. The 

 particles suspended in a fluid also wander under the influence of the 

 electric current, and indeed to the anode or cathode, according to the 

 nature of the fluid and the particles. This phenomenon is called cata- 

 phoresis. Such movements have also been found in colloidal solutions. 

 According to BILTZ/ in dialyzed aqueous solution, the colloid metallic 

 hydroxides wander to the cathode, and the other colloids (metals, 

 metallic sulphides, acids) wander to the anode. The colloid particles 

 in water are therefore probably electrically charged, hence the nega- 

 tively charged wander to the anode and the positively charged to the 

 cathode. Dialyzed protein solution shows no cataphoresis. The addi- 

 tion of acid or alkali gives to the protein a positive or negative charge 

 respectively, hence an alkaline solution wanders to the anode and an acid 

 solution to the cathode (HARDY, 2 PAULI 3 ). 



Precipitation of the Colloids. 



The colloids can be separated from their solutions in various ways. 

 Many colloidal solutions are so unstable that they flock out after a 

 time without the addition of anything (silicic acid, metallic hydroxides). 

 Certain colloids appear as flocculent precipitates on heating their solu- 

 tions (certain proteins, see Chapter III). Others solidify on cooling 

 from hot concentrated solutions, as semisolid forms, so-called jellies 

 or hydrogels, containing considerable water (glue, starch, agar). 



On eVaporating the hydrosols at ordinary temperature we obtain 

 a residue which ZSIGMONDY divides into reversible and irreversible col- 

 loids, according whether they are again soluble in water or not. 4 Accord- 

 ing to this definition starch, dextrin, agar, gum, protein belong to the 

 reversible colloids while colloidal silicic acid, stannic acid, colloidal metallic 

 hydroxides and sulphides, and the pure colloidal metals belong to the 

 irreversible colloids. The former are relatively non-sensitive toward 

 the addition of electrolytes, while the latter flock out on the addition 

 of the smallest quantity of electrolyte, and indeed again in an irreversible 

 form. This classification stands in accord with what was given above 

 (page 37), as the reversible colloids coincide in a measure with the 

 hydrophile colloids and the irreversible with the suspension colloids. 



1 Ber. d. d. chem. Gesellsch., 37, 1095, 1904. 3 Hofmeister's Beitrage, 7, 531, 1906. 



2 Journ. of Physiol., 24, 288, 1899. 4 Zur Erkenntniss der Koll., page 21. 



