METHODS OF COLLOID RESEARCH 



105 



Precautions: For these experiments there should be utilized only absolutely pure 

 gelatin or agar which has been dialyzed at least two or, preferably, four or five days 

 in cold running water. Commercial gelatin always contains, besides certain other 

 impurities, sulphurous acid which is used as a bleach in its manufacture, and as a 

 result the gelatin reacts acid to litmus. It may be completely freed from the 

 acid by neutralization with NaOH followed by sufficiently prolonged dialyzation 

 in running water. The dried gelatin is weighed, wrapped in linen or mull and 

 placed in a trough of running water. After purification the swollen gelatin is 

 very carefully removed from the cloth and weighed to see how much water it has 

 taken up. By the addition of, or the evaporation of water, the jelly is brought 

 to the desired concentration and filtered through a jacketed filter. This is 

 usually a sufficiently accurate method. For absolutely exact investigation, a 

 measured quantity of the moist gelatin must be weighed before and after it has 

 been dried at 105 C. If working with other than pure aqueous solutions, such 

 as with substances which require physiological salt solution to dissolve them, 

 the gelatin or agar must contain the required amount of salt, if, for instance, the 

 diffusion of globulin solution is desired. Since diffusion experiments with col- 

 loids always extend over a considerable time, the test tubes must be closed with 

 paraffined corks or rubber stoppers. 



In such cases quantitative measurements may be made with a 

 ruler, by placing the zero at the meniscus of the jelly or by means 

 of cathetometer. Tubes with an engraved scale as arranged by 



FIG. 21. Diffusion tube. FIG. 21a. Dabrowski's 



diffusion apparatus. 



STOFFEL-PRINGSHEIM* (see Fig. 21) are convenient. The graduated 

 tube is filled with jelly and the solution is poured into the extensions 

 which are ground on water tight. For accurate measurements the 

 same rules are used as in similar physical measurements. 



If the diffusion into the jelly is not associated with a visible change, 

 the jelly is removed from the glass by placing it in hot water until 

 the periphery melts, so that the cylinder of jelly may be gently 

 pushed out of the tube. The jelly cylinder is then cut into layers 

 of measured thickness which are studied by chemical, biological or 

 animal experiments as to their content of diffused substance. In 

 this way Sv. ARRHENIUS* and TH. MADSEN have determined the 

 diffusion constants of diphtheria toxin and antitoxin and of tetanoly- 

 sin and antitetanolysin. 



