Physiology and biochemistry in modern medicine . e; or, when it is of high spe-cific gravity, the tube of gelatin, etc., is placed mouth downwards inthe solution. In the case of colloidal solutions very little if any diffu-sion into the gelatin or agar will occur, even after several days; whereastrue molecular solutions will diffuse for a considerable distance. Whencolored solutions are used, the diffusion can readily be recognized byvisual inspection (see Fig. 13), but when they are colorless, the presenceor absence of diffusion must be determined by removing the columnof gelatin or agar and
Physiology and biochemistry in modern medicine . e; or, when it is of high spe-cific gravity, the tube of gelatin, etc., is placed mouth downwards inthe solution. In the case of colloidal solutions very little if any diffu-sion into the gelatin or agar will occur, even after several days; whereastrue molecular solutions will diffuse for a considerable distance. Whencolored solutions are used, the diffusion can readily be recognized byvisual inspection (see Fig. 13), but when they are colorless, the presenceor absence of diffusion must be determined by removing the columnof gelatin or agar and dividing it into slices of equal size, which arethen examined chemically for the substance in question. A further test is afforded by the failure of colloids to diffuse throughmembranes (dialysis). This was the method originally used by ThomasGraham to distinguish between molecular and colloidal solutions. Thesolution 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 glassvessel or slide vised 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 sacshave been adopted. These consist of pig or fish bladders or of col-lodion sacs. The latter are made by placing some collodion dissolvedin ether in a test tube, which is then tilted so that the collodion runsout except for a thin layer which remains adherent to the walls. Whenthe 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 sac containing the colloidal solution is then suspended in wateror some of the solvent used in preparing the colloidal solution, c
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