PRESERVATION OF FISH BY SALT. 3 



eral substance equally soluble would preserve in the same way that 

 salt does, but salt happens to be the only one that the human palate 

 and stomach will tolerate. 



HOW SALT EXTRACTS WATER. 



At the risk of appearing verbose the writer undertakes to elucidate 

 the principles that govern osmosis, because osmosis is nearly the 

 whole principle of salting fish. Without a knowledge of osmosis 

 people may salt fish successfully by rule, but without such a 

 knowledge it is quite impossible to understand the process. 



If a thin animal skin or membrane separates two liquids and if 

 the liquids are alike and of the same concentration, nothing happens. 

 But if they are unlike and of different concentration, one or the 

 other or both of the liquids will pass through the skin to the other 

 side. This passage through the skin or membrane is called osmosis. 

 Just what components pass through the membrane, in what direc- 

 tion, and how much depend on many circumstances. For the pur- 

 poses of salting fish water is always the liquid, plus whatever is 

 dissolved in the water. The dividing membrane is the skin of the 

 fish and the membranous inclosures of the microscopic cells of which 

 the substance of the fish is composed. We thus have water and salt 

 outside, cell membrane between, and fish juice, or protoplasm, in- 

 side, and we desire to loiow what will happen and how we can in- 

 fluence the process to suit our needs. The quantity and direction 

 of flow through the skin or cell membrane will depend on (1) the 

 nature of the dividing membrane, and (2) the nature and quantity 

 of the substances dissolved in the water on each side. 



The nature of the dividing membrane will be considered first. 

 Almost any substance can be made into a thin film or membrane. 

 Such things as glass, tin-foil, and mica may be exceedingly thin, 

 but are totally impermeable and therefore uninteresting in the pres- 

 ent connection. But other membranes or films, such as parchment 

 paper, gelatin films, animal bladders, and goldbeater's skins are 

 permeable to a greater or smaller degree. Suppose pure water were 

 on one side of a membrane and water containing dissolved salt on 

 the other. If the membrane is perfectly permeable to all constitu- 

 ents, water will pass through to the salt solution and salt will pass 

 through to the water, and these movements will continue until the 

 two sides are alike and then stop. It is always the tendency for 

 the two liquids to come to equilibrium, and they would do so if the 

 membrane were perfectly permeable. Nearly all membranes, how- 

 ever, permit a freer flow of the solvent, in this case water, than they 

 do of the solute (that which is dissolved) , in this case salt. 



If the membrane permits the water to flow but absolutely prevents 

 passage of a dissolved substance, the membrane is said to be semi- 

 permeable. In the example taken above, of pure water on one side 

 and salt solution on the other, if the membrane were semipermeable 

 then the water would pass through to the salt solution, but the salt 

 could not get through to the water. The level of the pure water 

 would fall and that of the salt would rise. The difference in liquid 

 level would exert a pressure called osmotic pressure. Ideally semi- 

 permeable membranes are not realized in nature, though some of the 



