Taylor. — Preservation of Fish by Salt 125 



It is found that those which never pass through are also those 

 which on drying out do not crystallize, but shrink to a tough 

 mass. They are called colloids; examples of them are glue, 

 albumen, gelatin, and soap. The smallest possible particle of 

 these substances is comparatively large, too large, we may 

 imagine, to go through the texture of the membrane. They 

 are not only large of molecule but complex in structure. The 

 bulk of animal bodies consists of colloids called proteins, dis- 

 solved in water. The other class of substances, those that may 

 pass through membranes, are the crystalloids — substances 

 which, on drying out, crystallize in regular geometrical shapes. 

 Examples of this class are salt, sugar and like substances. It is 

 not to be supposed, however, that all crystalloids will pass with 

 equal facility through any given membrane. Nearly all mem- 

 branes are in some measure selective of particular crystalloids. 

 The ideal semi-permeable membrane permits none to pass, but 

 as membranes degenerate from ideal semi-permeability to com- 

 plete permeability, they permit more and more of these dis- 

 solved things to pass through. 



The phenomena of osmosis having been briefly reviewed, one 

 may readily perceive the importance of applying the principles 

 to the salting of fish. Salt is brought in contact with the ex- 

 terior of the cell, it dissolves in some of the moisture, forming 

 a saturated solution. This solution is separated from the con- 

 tents of the cell by a cell membrane, which is more or less 

 semi-permeable. Water passes out of the cell to the salt and 

 the processes of decay are stopped because of insufficiency of 

 water. The membrane, not being absolutely semi-permeable, 

 permits some salt to enter, and the fish remains salty. The 

 contents left in the cell are proteins or the valuable food ele- 

 ments of the fish which, being colloids, are not permitted by 

 the cell-membrane to pass out. Thus, the fish is preserved. 



When the time comes to eat the fish the process is exactly 

 reversed. The fish is bathed in pure water. The cell contents 

 are more concentrated than the exterior, so water passes in. 

 The cell-membrane is to some extent semi-permeable, so the 



