Mr. T. Graham on Liquid Diffusion applied to Analysis. 371 



is dialysed, the potash is quickly reduced to the proportion of 

 about 9 per cent., which forms a neutral compound. If an excess 

 of acetic acid now be added, the whole potash is soon diffused 

 away, and pure soluble caramel remains on the dialyser. Even 

 carbonic acid will carry away the potash. 



The extremely low diffusibility which has been assigned to 

 caramel in former Tables, belongs to that substance as last 

 described, — the brown intermediate substances which accompany 

 it in crude caramel being considerably more diffusive, although 

 they, again, are much less diffusive than any variety of crystalli- 

 zable or uncrystallizable sugar. When the molasses of the cane- 

 sugar are diffused, much the greater portion of the colouring- 

 matters remains in the dialyser. 



With the parchment-paper septum the fluid caramel appeared 

 even less dialysable than gum, the diffusate in twenty-four hours 

 from a 2 per cent, solution of the former being '009 gramme 

 only, while that of the latter was "013. Caramel may be stated) 

 approximately, to be 600 times less dialysable than chloride of 

 sodium, and 200 times less so than sugar. Hence liquids 

 coloured with caramel, such as porter and coffee, may be dialysed 

 for a day with the passage of very little colouring-matter. 



Before leaving caramel, the analogy may be referred to which 

 the insoluble form of that substance presents to coal. Carame- 

 lization appears the first step in that direction — the beginning 

 of a colloidal transformation to be consummated in the slow lapse 

 of geological ages. 



Albumen. — The purification of albumen is effected with much 

 advantage upon the dialyser. The solution of egg-albumen is 

 mixed freely with acetic acid and then dialysed. The earthy 

 and alkaline salts are speedily got rid of, and in three or four 

 days the albumen burns without leaving a trace of ash. Although 

 the acetic acid used in the process appears to diffuse off entirely, 

 albumen prepared in the manner described has a faint acid re- 

 action. It also coagulates milk when mixed with the latter and 

 heated. Albumen so prepared retains its constituent sulphur. 



The passage through parchment-paper of pure albumen pre- 

 pared by the unobjectionable process of M. Wurtz is so slow, 

 that several days are required to produce a sensible result- 

 Thus the diffusate from a solution of 2 grammes of albumen in 

 50 grammes of water was 0*052 gramme in eleven days, which 

 gives 0-005 gramme in a single day. Albumen, then, appears 

 to be about 2^ times less dialysable than gum, and 1000 times 

 less so than chloride of sodium. 



Even combination with an alkali does not appear to enable 

 albumen to pass through the colloid septum. To half a gramme 

 of pure albuminic acid dissolved in 50 grammes of water, "05 



2C2 



