104 COLLOIDS IN BIOLOGY AND MEDICINE 



stand in a perfectly quiet place; instead of portable water baths, 

 incubators should be used, or if these are not placed so as to be 

 free from vibration, the experiments are kept by preference in a 

 cellar. 



These diffusion experiments are extremely difficult, but may yield 

 absolutely perfect results, as has been shown by the researches of 

 R. O. HERZOG and H. KASARNOWSKL* These investigators de- 

 termined the diffusion coefficients for albumin and a number of 

 enzymes (see p. 54 and p. 190), from which it could be determined 

 that they were simple substances. On the other hand it could be 

 shown that clupe'in sulphate, trypsin and pancreatin were mixtures 

 of different substances. Some of them showed various diffusion 

 layers of dissimilar composition (various percentages of N, in clupei'n), 

 and with other mixtures, products of different origin showed different 

 coefficients of diffusion (trypsin, pancreatin). 



Diffusion in a Jelly. A jelly acts like a membrane. It has the 

 advantage over a membrane that its thickness may be varied at 

 will, but the disadvantage that it is usually impossible to obtain the 

 diffused substance in pure form so that the diffused substance must 

 be examined in association with the jelly. Union or adsorption 

 between the jelly and the substance under examination is a more 

 disturbing factor than in the case of a membrane. Diffusion in a 

 jelly has the great advantage over the diffusion in fluids, above de- 

 scribed, that it is not disturbed by currents or the almost unavoidable 

 shaking during the experiment, or while samples are being taken. 



The experiments are generally performed as follows : A test tube is 

 filled one third to one half full with a very dilute jelly (2 to 5 per cent 

 gelatin). After the jelly solidifies, the solution to be investigated is 

 poured upon it, and the test tube is placed in an ice box. After a 

 longer or shorter time (days, weeks, months) some of the substance 

 will have diffused into the jelly. The supernatant fluid is now poured 

 from the jelly, which is washed with a suitable fluid, water, physio- 

 logical salt solution or the like. The jelly is now examined to deter- 

 mine the course of diffusion, taking the elapsed time into consideration. 

 Gelatin and agar are used as jellies. In many cases, inspection shows 

 the extent to which the fluid has diffused into the jelly (e.g., with dye- 

 stuffs, indicators or precipitation reactions, the results may be seen). 

 For example, gelatin which has been mixed with red blood corpuscles 

 may have tetanolysin layered above it. By the extent of the hemo- 

 lysis it is determined how far the tetanolysin has penetrated. H. 

 BECHHOLD* 2 mixed a jelly with goat-rabbit serum and layered above 

 it a solution of goat serum. The appearance of a white ring showed 

 the distance that the precipitin had penetrated. 



