Mr. J. J. Coleman on Liquid Diffusion. 3 



Although from many points of view, and especially that of 

 the chemist, most valuable and important deductions can be 

 made from this class of experiments, some of which will be 

 referred to in a later part of this paper, it must be admitted 

 that, from the physicist's point of view, all these experiments 

 are vitiated from the fact that the diffusions should commence 

 with the solid salt or the anhydrous acid or alkali, and end in 

 an atmosphere of pure water. With Graham's earlier expe- 

 riments a saline or acid solution, the strength of which was 

 gradually diminishing, was diffused into a weaker saline or 

 acid solution which was constantly increasing ; and even with 

 his latest method of " Jar diffusion " it is practically impossible 

 to work with syphons in long columns of water such as are 

 necessary to get a pure water atmosphere above, the salt being 

 diffused. I have therefore turned my attention to improved 

 methods of working, and have devised apparatus, illustrated by 

 Plate I. fig. 1*. This apparatus allows of a very concentrated 

 solution of the substance under examination being admitted 

 at any desired rate of speed underneath a column of pure 

 water of any desired length, and, further, of the liquids being 

 drawn off in regulated quantities at the end of the time of 

 diffusion. No doubt, with solid crystalline salts, theory in- 

 dicates that diffusion should commence from the salt ; but 

 practice determines the fact that, with solid salts, air-bubbles 

 are a difficulty; and, moreover, there are a very large number 

 of salts that are not crystalline at all, and some that do not 

 exist in the solid state, such as MgCl 2 . 



Returning to the description of fig. 1, B B represents a 

 Mohr's burette 500 millim. long and 15 millim. diameter 

 provided with a glass stopcock. 



The stopcock being closed, this burette is nearly filled with 

 pure water, and is then connected by india-rubber tubing 

 with an apparatus by means of which the air above the water 

 can be slightly rarefied, such as an air-pump or an aspirator, 

 or, by what I find most convenient, an open glass tube C C 

 standing in a jar of water. On opening the stopcock of the 

 burette, about half its water escapes, say, to the level (a) 

 (rarefying the air above), upon which a column of water 

 rushes up the tube C C, say to x. The stopcock being now 

 closed, the tube C C is raised and clipped in such a position 

 that the columu of water in the tube C C is much longer than 

 the water-column in the burette, by which means the air above 

 the water in the burette is sufficiently rarefied to admit of 

 the concentrated saline or other solution being sucked up 



* A description of this apparatus and the principal results detailed in 

 this paper were communicated to the Philosophical Society of Glasgow, 

 16th April, 1886. 



B2 



