THE CELL 55 



these are transformed inside the cell, i.e. substances 

 within the cell, do not again escape from it. The 

 experiment we have just performed fully confirms this. 

 In fact the liquid blackens only inside the collodion 

 bladder ; outside, it is as colourless as water. This 

 would not have happened could tannin, or its compound 

 with the iron salt, ink, pass through the cell-wall. To 

 verify this let us perform the converse experiment. 

 Let us pour some iron salt into the cell and some tannin 

 into the outer vessel. In a few moments black streams 

 will be noticed in the outer vessel, and in the end the 

 whole liquid in it will become so black that the bladder 

 will be invisible (3, fig. 19). Let us take it out of the 

 vessel the solution inside of it is as colourless as it 

 was at first. Without doubt it is only the iron salt 

 which passes freely through the membrane, with equal 

 ease in either direction ; but neither tannin, nor its 

 compound with iron, can pass through it. It follows that 

 two kinds of substances exist : some of them are capable 

 of passing through the membrane of the cell, others are 

 not ; the iron salt serves as an illustration of the 

 former kind and tannin of the latter. 



Indeed these two substances may serve as types of 

 two great classes of chemical bodies. Those of the one 

 class pass easily through vegetable or animal membranes; 

 those of the other pass with difficulty. We have noticed 

 in speaking of the diffusion of liquids that some diffuse 

 more quickly, others more slowly ; some are more 

 mobile, others less. We may now add that those 

 substances which diffuse slowly are precisely those that 

 pass still more slowly through membranes. Chemists 

 call substances of the former class crystalloids, since they 

 are all capable of crystallisation ; substances of the latter 

 class they call colloids, i.e. gum-like substances ; these 

 are all incapable of crystallisation. 



We have here at once an explanation of our experi- 

 ment, and a general key to the phenomena which take 



