82 SOLUTIONS AND PROTOPLASM [Cn. Ill 



merited upon was due alone to the osmotic action of the salts. 

 To this conclusion he was led by an unfortunately devised 

 experiment. He compared the action of several pairs of solu- 

 tions, one of the members of the pair being a salt, and the 

 other member sugar, the dissolved substance of each having 

 the same gross weight. In all cases the action of the salt 

 was the more powerful. But this is what we should expect 

 upon the theory that death is caused by osmosis, since the 

 osmotic index of sugar is far lower than that of any of the salts 

 with which comparison was made. 



Let us now ascertain the relation between the resistance 

 period and the "osmotic index." To determine the relative 

 resistance periods for any species in the different salts, we may 

 take as our unit the average resistance period to all the salts, 

 and express the separate resistance periods in terms of that 

 unit. To determine the osmotic index, we divide the isotonic 

 coefficient by the molecular weight. The resistance periods 

 will vary inversely as the osmotic indices. For the salts, NaCl, 

 MgCl 2 , MgSO 4 , the reciprocals of the osmotic indices are : 19.6, 

 23.8, 58.8 ; and the mean relative resistances are : 19, 63, 217. 

 From this comparison it is seen that while the reciprocals of 

 the osmotic indices increase roughly from 1 to 3, the relative 

 resistance period increases from 1 to 11 ; or the resistance 

 period increases more rapidly than the reciprocals of the osmotic 

 indices, and roughly as the squares of those reciprocals. 



At about the same time with PLATEAU'S work was published 

 that of BERT ('71). The work of the latter was done chiefly 

 upon fresh-water fishes ; incidentally, upon frogs and some 

 fresh-water Arthropoda. These were plunged directly into 

 sea water, and their resistance periods determined. Some 

 species showed an extraordinary variability in their resistance 

 period ; sticklebacks (Gasterosteus leiurus) from the same 

 locality (about Paris) resisting for from 2 hours to 1 month 

 or more. 



A decided advance was made by BERT in observing that the 

 resistance period varies with the temperature ; thus, the 

 European minnow (Phoxinus leevis) died in sea water - 



at 9 C. in 30 minutes, at 22 C. in 14 minutes, 



at 14 C. in 25 minutes, at 28 C. in 9 minutes. 



