SELECTIVE PERMEABILITY 79 



occurs in the organism we can not defi- 

 nitely say. 



As soon as the cell dies, its power of 

 selective absorption disappears. Sub- 

 stances can then diffuse in and out of the 

 protoplasm without hindrance (unless in- 

 terfered with by the non-living cell wall, 

 as happens in some cases). 



It follows that if we can measure the 

 permeability of the cell to various sub- 

 stances, we can tell whether it is alive 

 or dead. One way of doing this is to put 

 the cell into a solution of a dye. Cer- 

 tain dyes will not enter as long as the 

 cell is alive while others enter and be- 

 come stored up inside, reaching a higher 

 concentration than outside. This proc- 

 ess obeys a definite mathematical law as 

 has been shown by Miss Irwin. In the 

 dead cell this storage of the dye does 

 not occur to so great an extent. 



Another way is by sending an elec- 

 tric current through the cell; in a salt 

 solution the current is carried by the 

 charged particles derived from the salt 



