BIO-CHEMICAL JOURNAL 



3 . Reversibility 



The question as to how far the process is a reversible one is of 

 some theoretical interest. It has been already mentioned that if the 

 temperature is raised up to iooC., a part of the dye is fixed in the 

 paper, so that under certain conditions, the reversibility is not complete. 

 At ordinary temperature it appears complete. If a circle of paper be 

 dyed with congo-red, rinsed with water and then placed in water along 

 with another similar circle of paper, it will be found that after some 

 time the two pieces of paper will be of the same depth of colour. At 

 the same time, as already pointed out at the beginning of this paper, 

 to completely extract all the dye from a piece of paper needs an 

 enormous number of repeated changes of water. 



As regards temperature reversibility is apparently complete. 



Experiment : Two flasks containing congo-red and filter-paper 

 were placed, one at 50, the other at 11 ; next day both flasks were 

 placed in a thermostat at 25, when equilibrium was attained after 

 about 30 hours, the colorimeter readings were : 



From 50 - 53 % 

 " - 57 % 



Again, if sufficient time be allowed for equilibrium to be reached, a 

 preparation may be alternately warmed and cooled, and nearly the 

 same readings obtained at the same temperature. It appears, however, 

 that after several days contact the dye becomes partly fixed. For 

 example, in one case the reading at 38 was 66 per cent., it was then 

 cooled to 13 with a reading of 48 per cent. ; on again warming to 38, 

 50 per cent, only was reached, and on cooling again to 13, after two 

 days the value became 25 per cent. 



Gelatin, also, shows fairly complete heat reversibility as regards 

 its adsorbed electrolytes. At 1 1 C. a particular hydrogel had a specific 

 conductivity of 162 gemmhos, after being heated to 53 and again 

 cooled to iiC. its conductivity was 157*8 gemmhos. Another pre- 

 paration containing more electrolytes had an initial conductivity of 609 

 gemmhos, after heating to 55*5 and cooling again to 11, it became 

 542 gemmhos, or less than its original conductivity. Perhaps in the 

 first measurement equilibrium was not completely attained. 



