8 COLLOIDS IN BIOLOGY AND MEDICINE 



To recapitulate: The chief characteristic of sols is the large size 

 of their particles, 1 which are unable to pass through vegetable or 

 animal membranes. The natural size of the particles accounts for 

 this in the case of natural sols, while in artificial sols it is due to the 

 defects of our technic, which hitherto has not permitted our prep- 

 aration of such substances in molecular, or even approximately 

 molecular subdivision. 



This criterion is only valid for extreme cases. Between the un- 

 doubted colloids, e.g., albumin, and the undoubted crystalloids, e.g., 

 amino acids, there are all kinds of transition forms, which pass 

 through the same membranes more or less rapidly, e.g., albumoses 

 and peptones. There is, indeed, no sharp line of demarcation be- 

 tween colloids and crystalloids. 



Gels. 



It might be inferred from the nomenclature (colloids and crys- 

 talloids) that the main distinguishing feature was the ability or 

 inability to crystallize. It is a fact that most crystalloids, i.e., sub- 

 stances which pass through membranes, are crystallizable, whereas 

 most colloids are not able to form crystals when they separate from 

 solution. However, this is not a radical difference, since egg albumin 

 and hemoglobin which are undoubted colloids may be obtained in 

 beautiful crystals; and I have further established by ultrafiltration, 

 the colloidal nature of the solutions of the alkaline salts of the fatty 

 acids (e.g., oleic acid) which also form good crystals. Colloids usu- 

 ally separate from their solution in unformed masses called gels. 



If the solid phase be separated from crystalloid solutions, it may 

 form either crystals or a slightly or even an entirely amorphous pre- 

 cipitate. Cuboidal crystals separate from a solution of common salt 

 on evaporation or addition of alcohol, and crystals of Na^SCX -f- 

 10 H 2 separate from solutions of Glauber's salt (sodium sul- 

 phate). A white precipitate of barium sulphate, which has hardly 

 any definite form, separates from a sodium sulphate solution upon 

 adding barium chlorid. A substance of constant chemical com- 

 position especially as regards water content is obtained if the im- 

 purities, especially the extraneous water, are removed by nitration 

 from the crystals or precipitate. To return to our example: the 

 sodium chlorid crystals and the barium sulphate are water-free, 

 whereas the sodium sulphate contains 10 molecules of water to one 

 molecule of Na^SO^ but it is water-free above 33 C. 



1 By the passage through a membrane, I mean especially passage by means 

 of dialysis. In many cases we may substitute ultrafiltration, provided ultrafilters 

 of sufficient tightness are employed (see p. 102). 



