COLLOIDS 53 



are half filled with a 5 per cent solution of pure gelatin or a 1 per cent 

 solution of pure agar, and, after the jelly is set, the solution under 

 examination is poured on the surface; or, when it is of high spe- 

 cific gravity, the tube of gelatin, etc., is placed mouth downwards in 

 the solution. In the case of colloidal solutions very little if any diffu- 

 sion into the gelatin or agar will occur, even after several days; whereas 

 true molecular solutions will diffuse for a considerable distance. "When 

 colored solutions are used, the diffusion can readily be recognized by 

 inspection (see Pig. 13), but when they are colorless, the presence or 

 absence of diffusion must be determined by removing the column of 

 gelatin or agar and dividing it into slices of equal size, which are 

 then examined chemically for the substance in question. 



A further test is afforded by the failure of colloids to diffuse through 

 membranes (dialysis). This was the method originally used by Thomas 

 Graham to distinguish between molecular and colloidal solutions. The 

 solution under examination is placed in a dialyzer, which is then im- 

 mersed in a wide vessel containing the pure solvent. The older forms 



Fig. 12. Ultramicroscope (slit type) for the examination of colloidal solutions. The arrange- 

 ment of diaphragms, etc., in this form removes the absorptive effects of the surfaces of the glass 

 vessel or slide used to contain the colloidal solutions. 



of dialyzer consisted in general of a bell-shaped glass vessel closed be- 

 low with parchment paper, but more recently so-called diffusion sacs 

 have been adopted. These consist of pig or fish bladders or of col- 

 lodion sacs. The latter are made by placing some collodion dissolved 

 in ether in a test tube, which is then tilted so that the collodion runs 

 out except for a thin layer which remains adherent to the walls. When 

 the collodion has set, the sac can be removed after loosening it by allow- 

 ing a little water to flow between the sac and the walls of the test tube. 

 The sac containing the colloidal solution is then suspended in water 

 or some of the solvent used in preparing the colloidal solution, care 

 being taken that the menisci of the fluids inside and outside of the sac 

 stand at the same level. Sometimes, especially when collodion sacs are 

 used, some colloid may at first diffuse through, but if the outer fluid 

 (the dialysate) is renewed and the dialysis allowed to proceed, this 

 ceases. 



