6 COLLOIDS IN BIOLOGY AND MEDICINE 



microscopically, may last months and even years. Only about a 

 decade ago there was an animated discussion as to whether the known 

 inorganic colloids such as colloidal silver, gold, arsenic sulphid, 

 Prussian blue, etc., were suspensions or true homogeneous solutions. 

 Some evidence was against their being considered homogeneous 

 solutions; other evidence, however, favored this view. Microscopi- 

 cally, they seemed entirely homogeneous, and they could not be 

 separated from their solvents by mechanical means (filtration or 

 centrifugation). 



It was only by means of the ultramicroscope, invented by H. 

 SIEDENTOPF and R. ZSIGMONDY (1903), that it was convincingly 

 shown that they were suspensions and not homogeneous solutions. 



After this point was settled, the further question arose as to 

 whether gelatin, albumin sol and like substances were to be con- 

 sidered true solutions. Under the ultramicroscope, they also showed 

 tiny particles, which, however, were by no means as numerous as 

 might have been expected. Evidently most of them were invisible, 

 and it was uncertain whether this was due to conditions of refrac- 

 tion, or whether the larger part of these substances was in true solu- 

 tion. This question was settled by the method of ultrafiltration 

 invented by H. BECHHOLD in 1906. By a sufficiently impermeable 

 jelly filter (ultrafilter) , that is, by a purely mechanical process, he was 

 able to separate solutions of albumin, gelatin, enzymes, toxins, etc., 

 from their aqueous solvent. Not only did albumin, gelatin, etc., 

 prove to be suspensions or emulsions, but in addition, substances 

 whose true solubility had hardly been questioned, e.g., most of the 

 albumoses,. and even dextrin whose molecular weight had been placed 

 at about 1000, and which had practically been classified as a crys- 

 talloid. 



It is also possible to accomplish such a separation by means of 

 centrifugation. By centrifugation at 6,000 revolutions per minute, 

 H. BECHHOLD separated colloidal silver sols (collargol) into coarser 

 and finer particles. H. FRIEDENTHAL has recently constructed 

 centrifuges turning from 10,000 to 30,000 revolutions per minute, 

 by means of which he can separate the casein from cows' milk. 



We must here refer to the definition for "homogeneous" and 

 "homogeneous solution" given by H. W. BAKHUIS ROOZEBOM, whose 

 premature death we lament: "We call a system homogeneous, if all 

 its mechanically separable particles possess the same composition and 

 the same physical properties. Therefore, this homogeneity of con- 

 stitution exists in a well-mixed liquid only because of the smallness 

 of molecules and the coarseness of our means of observation." 



We cannot speak of a definite solubility in respect to suspensions 



