4 Mr. J. J. Coleman on Liquid Diffusion* 



underneath the water by cautiously opening the stopcock. A 

 column, say, 100 millim. deep can be drawn into the burette 

 under a depth of water of 200 millim., which latter can be 

 further added to by filling up the burette cautiously by a long- 

 legged pipette discharged on a cork float. This, however, is 

 seldom necessary except with very diffusive substances, such 

 as hydrochloric acid, which barely reach a height of 200 

 millim. in 21 days. To prevent leakages, an indiarubber 

 cap is slipped over the point p of the burette. At the end of 

 the diffusion -time the liquid is very slowly run off until the 

 level where the water originally joined the saline solution is 

 reached, after which equally-measured sections are carefully 

 removed and reserved for further examination, each section 

 being, say, 25 millim. deep. The results are conveniently 

 calculated in percentages upon the salt or other substance 

 contained in the bottommost layer, where diffusion commenced. 

 It was thought at first that some errors might arise from 

 adherence of the saline solutions to the inside of the burette, 

 down which the upper layers had to pass before estimation. 

 This proved, however, not to be the case to any serious or 

 appreciable extent. Most diffusions are carried on until at 

 any rate 1 per cent, of the salt rises 50 millim. ; and it was 

 found that sulphuric acid of 1*2 specific gravity, admitted in 

 the burette under water coloured blue with litmus and then 

 withdrawn carefully, had so little effect on the water 25 mil- 

 lim. above that it came out of the stopcock barely reddened, 

 and the water lying 50 millim. above the acid came out quite 

 blue. Corroborative experiments were made with saturated 

 cupric sulphate and ammonia as an indicator. 



Hydrochloric acid being the most diffusive substance ex- 

 amined by Graham, comparative experiments were made with 

 this compound at temperatures of 12 0, 5 Centigrade, the 

 results of which are shown in the curves, fig. 2. 



Fig. 3 shows in the form of curves the results of some 

 comparative experiments made for a period of twenty days at 

 a temperature of 12 0, 5 C, with the following substances : — 



1. Sulphuric acid containing 20 per cent, anhydride. 



2. Hydrochloric acid „ 20 „ „ 



3. Nitric acid „ 20 „ „ 



4. Potassic hydrate „ 20 „ „ 



5. Sodic hydrate „ 20 „ „ 



6. Ammonia solution of *880 specific gravity. 



7. Magnesic sulphate . saturated. 



8. Sodic chloride . . . ditto. 



The ammonia solution was floated on the water, the rest 

 admitted under the water, and, with the exception of the 



