124 SLOW DIFFUSION. [CH. V 



a long funnel run in very slowly and carefully a stratum 

 of concentrated solution of potassium bichromate, which 

 accumulates at the bottom of the jar. 



It will be seen that the colour spreads to the upper 

 strata with extraordinary slowness. The chief physio- 

 logical interest of the result is that it serves to suggest 

 the value, to the living cell, of protoplasmic circulation. 



(144) Relation of membrane to diffusing fluid^. 



A dialyser made of vegetable parchment is filled with 

 a 1 per cent, solution of di-sodic phosphate coloured with 

 methylene blue, and is placed in distilled water ; after some 

 hours the blue colour is visible in the water. If, however, 

 a precipitation membrane of calcium phosphate is pro- 

 duced in the wall of the dialyser, the methylene blue is 

 unable to pass. The precipitate is produced by immersing, 

 in 1 per cent, calcium nitrate, a dialyser filled as before 

 with 1 per cent, di-sodic phosphate coloured with methylene 

 blue. The importance of the experiment is to show that 

 by the formation of a precipitation membrane the osmotic 

 quality of the parchment is changed. 



(145) Absorption of methylene blue. 



It is interesting to note in connection with the last 

 experiment that methylene blue, as Pfeffer^ has shown, 

 can pass a living protoplasmic membrane. 



Two or three sprigs of Elodea are placed in about a 

 liter of tap water containing 0"0008 per cent, of methylene 



1 Taken from Detmer's Pmktikum, p. 96. 



'^ Vntersuchungen am dem Bot. histitut zu Tubingen, ii. p. 223. 



