The Origin of Osmotic Effects. 



95 



from the solutions by the grains — cane-sugar is nearly as active as common 

 salt. In like manner, not only are strong acids and most salts indiffusible 

 through the membrane covering the grain but also not a few non-electro- 

 lytes ; the membrane is slowly permeated by certain weakly oxygenated 

 organic acids, by salts such as mercuric chloride and cadmium iodide, by 

 iodine, by ammonia and by a number of non-electrolytes of low molecular 

 weight. 



The compounds which penetrate the membrane, whether electrolytes or 

 non-electrolytes, are all substances which attract water presumably only to 

 a minor extent and which exist to some extent in solution in an unhydrated 

 condition ; those which cannot penetrate it, on the other hand, probably all 

 form hydrates of considerable stability in solution. 



I picture surfaces generally, colloid surfaces in particular, as not merely 

 wetted by water but as more or less hydronated and hydrolated — using these 

 terms in the specific sense explained in No. VIII of my " Studies on 

 Solutions " ; that is to say, they are not merely wetted by water complexes* 

 but associated with hy drone, the simple fundamental molecule of which water 

 is composed. The intramolecular passages in a colloid membrane, if thus 

 hydrolated, would be guarded by the attached hydrone molecules ; molecules 

 in a solution bathing the membrane which attempted to effect an entry 

 through such passages, if hydrolated, would be seized upon and held back in 

 virtue of the attraction which the two hydrolated surfaces — that of the 

 membrane and that of the solute — would exercise upon one another. The 

 hydrolated passages, however, would be indifferent to molecules which were 

 not hydrolated — consequently, a substance such as acetic acid, of which 

 probably only a small proportion is present in solution in the hydrolated 

 state, would gradually pass through them. 



The apparently exceptional behaviour of trichloracetic acid, which must be 

 more fully if not more firmly hydrolated than acetic acid, is very striking and 

 may be taken as proof that the hydrolation must extend over a certain area 

 to secure protection against penetration ; it should be noted, however, that 

 the result is in accordance with the behaviour of the acid as a substitution 

 derivative. Hydroxy- and amino-acetic acids (glycollic acid and glycine), 

 which are far weaker acids, are nevertheless far less easily diffusible — pre- 

 sumably because not only the carboxyl group of the acid but also the adjacent 

 hydroxyl- or amino-group is hydrolated. The behaviour of glycol, C2H4(OH) 2 , 

 in comparison with that of alcohol, C 2 H 5 (OH), may be interpreted in a similar 

 manner. The concentrating effect exercised by the sugars has already been 



* Compare ' Chemical News,' Jan. 15 and 22, 1909, pp. 28 and 37; 'Science Progress,' 

 Jan., 1909, No. XI, p. 484. 



