333 



pencil penetrates f'artlicr into the solution. The colour of the diffused 

 light is white and, as in all other cases, the light, emerging perpen- 

 dicularly on the incident pencil, is highly j)olarized. 



In experimenting upon the TYNDALL-phenonienon I was guided by 

 the knowledge of the changes in gelatin-solutions, obtained in study- 

 ing the viscosity in the selfsame solutions. Before long I intend 

 to discucs this point at length. With a view to elucidate the exper- 

 iments reported in this paper, it is necessary to preface them with 

 a short survey of my experience just now alluded to. 



The properties of a gelatin-solution, whether this be a sol or a 

 gel, are, in a condition of equilibrium, completely determined by 

 the concentration, the temperature and the pressure. Most often, 

 however, the solution worked with is not in equilibrium. Therefore, 

 even under constant outward circumstances alterations are to be 

 noted in the solution, which bring it nearer to the equilibrium. This 

 approximation occurs rapidly at 60° or at a higher temperature, 

 when the equilibrium is established in a few minutes. Lower tem- 

 peratures slacken this process. At 40° it takes some days even ; at 

 20° an equilibrium cannot be noted even after three weeks and at 

 0° the progress of the process is imperceptible. 



In order to find the value of a variable in the equilibrium at a 

 given concentration, temperature and pressure, I took the equilibrium 

 to be the limit of all the changes that occur in the solution. 

 Therefore, the preceding treatment was conducted in such a way, 

 that when the desired temperature was reached, the value of the 

 variable was either too high or too low% so that while the tem- 

 perature remained the same, it was respectively reduced and 

 augmented. By prolonging the experiment we can get as near to 

 the equilibrium as we like. 



Now it appeared that the intensity of the Ti'NDALL-cone evinced 

 changes similar to those described here. We publish an experiment 

 in which the phenomenon was faint soon after the required tempe- 

 rature was reached and w^as gradually intensified; a 27o gelatin- 

 glycerin solution, after being heated for five minutes to 70^, was 

 at once cooled in cold water. As soon as the room-temperature was 

 reached pictures were taken at regular intervals of an hour. The 

 images obtained, showed that the intensity of the light had increased 

 continuously. In the meantime another change had taken place iji 

 the solution, viz. the viscosity had increased slowly and after 2 

 hrs the solution had been solidified; even then, however, the intensity 

 was increasing ; this was proved by an exposure made after 

 12 hours. 



