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. 



The following experiment is quoted from Detmer's 

 Praktikum : we have had practically no experience of the 

 method. 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 produced 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 phos- 

 phate coloured with methylene blue. 



(145) Absorption of methylene blue. 



It is interesting to note in connection with the last 

 experiment that methylene blue, as Pfeffer 1 has shown, 

 can pass a living protoplasmic membrane. 



Two or three sprigs of Elodea are placed in about a 

 liter of tap water containing O'OOOS per cent, of methylene 



1 Untersuchungen aus dem Bot. Institut zu Tubingen, n. p. 223. 



