90 PROTOPLASM 



eidos, semblance), because substances such as glue are typical 

 of them. 



No science has its beginning with the work of one man, any 

 more than has its subsequent development. It is claimed that 

 the French chemist Baudrimont studied colloids seventeen years 

 before Graham. But Graham gave us names for the substances 

 and their behavior, and names cling. 



Inability to pass through a parchment-paper membrane was, 

 then, Graham's test for the colloidal state. To put a substance 

 to such a test is to dialyze it ; and the membrane, with its accou- 

 terments, is the dialyzer. The dialyzer which Graham used was 



a very simple affair (Fig. 67), parch- 

 ment paper tied over one end of a 

 large-mouthed funnel. The colloid or 

 crystalloid is on one side of the mem- 

 brane, and pure water on the other. 

 Membranes for dialysis may be of a 

 ^ „„ ^ , , .. , number of kinds. All animal mem- 



FiG. 67. — Graham s dialyzer. .,1111 



branes, such as a pig s bladder, serve 

 well. Artificial collodion (celloidin) membranes are much used. 

 These may be cast in the form of sacks into which the solution 

 to be dialyzed is put; the sack is then corked and suspended in 

 water. All these membranes have one property in common — 

 they permit crystalloids (salts, etc.) to pass through but not 

 colloids (proteins etc.). Through their use, colloidal substances 

 may be freed of impurities of noncolloidal nature; thus, if it is 

 desired to free blood serum of salts, dialyzing for many days, 

 with repeated changes of water, will do it, as far as is experi- 

 mentally possible. The salts leave by diffusion, if their con- 

 centration within the dialyzer is greater than that without. The 

 process may be hastened by eledrodialysis. 



The simplest form of electrodialyzer is one in which one electrode 

 is placed within a parchment or collodion sack, and its mate 

 placed outside in the surrounding water. The electric field will 

 hasten the outward diffusion of those ions of a sign opposite to 

 that of the outside electrode. By reversing the current every 

 few minutes and changing the water, both cations and anions 

 will be alternately attracted by the outer pole. More efficient 

 is the electrodialyzer of Pauli (Fig. 68). The apparatus consists 

 of a central chamber, into which is put the substance to be 



