328 Dr. J. H. Long on the Diffusion of Liquids. 



a head of water accumulates as before. There seems to be no 

 limit to this cycle of operations : on one occasion they were 

 seen to take place during the whole of a day ; and at the same 

 time the amount of salt diffused was about one fourth of what 

 was expected. The same phenomena were several times ob- 

 served when, before filling the apparatus, the thorough cleans- 

 ing of the tube b was neglected. A small amount of greasy 

 substance at this point seemed to exert a very great disturbing 

 force, and if not guarded against could become a source of 

 serious error, as was the case in many of the earlier experi- 

 ments, which were thus rendered quite worthless. I have 

 described at some length the influence of excessive concen- 

 tration of solution and the influence of capillarity at b, but at 

 no greater length than the importance of this part of the sub- 

 ject demands. 



It was only by guarding most carefully against these sources 

 of error that constant results, and results comparable among 

 themselves when different tubes were used, could be obtained. 

 It has been seen that the height ge at which the solution 

 stands in i at the beginning of the experiment, and upon 

 which depends the amount of salt diffused, depends in turn, 

 not only on the concentration, but also to a certain extent on 

 the velocity of the water, and on the amount of the resistance 

 at b. By keeping these elements within certain limits con- 

 stant results can be obtained, even when different tubes are 

 used, as shown below. 



Thus far nothing has been said of the influence of tempera- 

 ture on the progress of diffusion. During the continuance of 

 the above experiments, most of which were conducted in the 

 cellar of the laboratory at Tubingen, the temperature remained 

 quite constant at 14°-15° 0. Experiments undertaken to 

 decide this point of the influence of temperature were not very 

 successful, and chiefly for the reason that it was found very 

 difficult to keep any other than the ordinary temperature con- 

 stant for the necessary length of time. The experiments were 

 arranged as follows: — In a wooden box with double walls, 

 used as an ice-chest, were placed two diffusion-cells with the 

 necessary water-flasks. The remaining space on the bottom 

 of the box was occupied by vessels containing pounded ice. 

 Two thermometers were hung in different parts of the box. It 

 was possible by this means to obtain a temperature of 10° C. 

 inside the box when that of the outside was 15° C. Bat the 

 difficulty lay in keeping this 10° constant long enough to 

 complete an experiment — that is, for four or five days. The 

 chest was not large enough to contain a great deal of ice; and 

 replenishing this from day to day disturbed the glasses always 



