90 COLLOIDS IN BIOLOGY AND MEDICINE 



teaching laboratories is the one originally described by GRAHAM. A 

 wide-mouthed salt bottle A (Fig. 9), with its bottom broken off, has 

 a pig or ox bladder or a piece of parchment bound about the neck; 

 thejDottle is placed with the membrane downward, in a vessel of 



water (B) 1 . The solution to be dialyzed 



is placed in the bottle. 



Precautions: Before the membrane is used, 

 it should be tested to see that it can be made 

 wet by water. Greasy animal membranes 

 must be rinsed with fresh water several times, 

 on both sides. To determine whether the 

 FIG. 9. A simple dialyzer. membrane leaks, a colored solution (e.g., a 



drop of colloidal silver, litmus, or water colored 



with hemoglobin) is placed in A and allowed to remain there several hours, 

 without putting any water in the outer vessel. Colored drops will pass through 

 at points of leakage (the margin where the membrane is bound should be espe- 

 cially watched) . To make sure that the drops are really colored solution and 

 not pure water, they should be absorbed by filter paper. 



In all dialysis experiments, the substance under examination may simulate 

 colloidal character by being bound or adsorbed by the dialyzing membrane. 

 Under these circumstances, if we wish to determine whether the solution under 

 examination contains colloidal substances, it is necessary to use the smallest 

 possible membrane with the largest available amount of substance. If too little 

 substance is used, it may all become bound by the membrane, and in spite of the 

 fact that it is not colloidal, none will be found in the dialyzer. But if so much 

 of the substance under investigation is used, that in spite of any possible ad- 

 sorption by the membrane, plenty still remains in the dialyzer, then an examina- 

 tion of the water outside will settle the question. 



In practice, GRAHAM'S dialyzing apparatus is not frequently em- 

 ployed, because it has a small dialyzing surface, and this is a great 

 disadvantage. 



In order to bring the largest possible surface into contact with 

 the surrounding water, there are used either whole pig bladders, ox 

 bladders, fish bladders, or the commercial parchment thimbles. 2 



I have had very good results with fish bladders (condoms) which 

 are very thin, uniform and elastic, though unfortunately they are 

 expensive. Parchment thimbles, referred to above, are recommended 

 for the dialysis of large quantities of solution. They may be obtained 

 in all sizes and in all lengths. The suspension of a fish bladder is 

 conveniently accomplished by pressing it between two glass rods 

 which are held together by rubber bands (cut from gas tubing) ; the 



1 With organic solvents, instead of water, alcohol, benzol, etc., must of course 

 be employed and the membrane (preferably collodion) is previously soaked in 

 these fluids. 



2 These are on sale at the "Vereinigten Fabriken fur Laboratoriums-bedarf " 

 Berlin, Scharnhorst Str. (The Kny Scherer Co., N. Y., are the American agents.) 



