THE CHEMISTRY OF COOKERY. 233 



upon it. This is better shown if the water be colored by a blue tinct- 

 ure of litmus, which is reddened by the acid. A red stratum indicates 

 the boundaries of the two liquids. Gradually the reddening proceeds 

 upward and downward, the whole of the water changes from blue to 

 red, and the acid becomes tinged. 



Graham worked for many years upon the determination of the 

 laws of this diffusion and the rates at which different liquids diffused 

 into each other. His method was to fill small jars of uniform size and 

 shape (about four ounces capacity) with the saline or other dense so- 

 lution, place upon the ground mouth of the jar a plate-glass cover, 

 then immerse it, when filled, in a cylindrical glass vessel containing 

 about twenty ounces of distilled water. The cover being very carefully 

 removed, diffusion was allowed to proceed for a given time, and then by 

 analysis the amount of transfer into the distilled water was determined. 



I -must resist the temptation to expound the very interesting results 

 of these researches, merely stating that they prove this diffusion to be 

 no mere accidental mixing, but an action that proceeds with a regu- 

 larity reducible to simple mathematical laws. One curious fact I must 

 mention, viz., that, on comparing the solutions of a number of differ- 

 ent salts, those which crystallize in the same forms have similar rates 

 of diffusion. The law that bears the most directly upon cookery is 

 that "the quantity of any substance diffused from a solution of uni- 

 form strength increases as the temperature rises." The application 

 of this will be seen presently. 



It may be supposed that, if the jar used in Graham's diffusion ex- 

 periments were tied over with a mechanically air-tight and water-tight 

 membrane, brine or other saline solution thus confined in the jar could 

 not diffuse itself into the pure water above and around it ; people 

 who are satisfied with anything that " stands to reason " would be quite 

 sure that a bladder which resists the passage of water, even when the 

 water is pressed up to the bursting-point, can not be permeable to a 

 most gentle and spontaneous flow of the same water. The true phi- 

 losopher, however, never trusts to any reasoning, not even mathemati- 

 cal demonstration, until its conclusions are verified by observations 

 and experiment. In this case all rational preconceptions or mathe- 

 matical calculations based upon the amount of attractive force exerted 

 between the particles of the different liquids are outraged by the facts. 



If a stout, well-tied bladder that would burst rather than allow a 

 drop of water to be squeezed mechanically through it be partially 

 filled with a solution of common washing-soda, and then immersed in 

 distilled water, the soda will make its way out of the bladder by pass- 

 ing through its walls, and the pure water will go in at the same time ; 

 for if, after some time is allowed, the outer water be tested by dipping 

 into it a strip of red litmus-paper, it will be turned blue, showing the 

 presence of the alkali therein, and, if the contents of the bladder be 

 weighed or measured, they will be found to have increased by the in- 



