Mabch 11, 1904.] 



SCIENCE. 



423 



wetted by the solution from which mole- 

 cules are adsorbed. In the investigations 

 here reported, not only were solids used 

 which were wetted by the aqueous solution, 

 but also paraffine, in the case of which a 

 different type of contact exists between the 

 solid and the solution. Perhaps no contact 

 exists in the sense of a relation so intimate 

 as to bring the molecules of the dissolved 

 substance into actual contact with the 

 paraffine. The contact surface between the 

 paraffine and aqueous solution may, per-, 

 haps, be regarded as comparable to a sur- 

 face film which bounds the contact of the 

 solution with air. Whether or not an 

 actual air film is present between the 

 paraffine and aqueous solution, the presence 

 of paraffine in solutions of the toxic agents 

 was seen to exert a marked effect, perhaps 

 as great as that of sand. It is probable, 

 therefore, that the generalization of the 

 physical chemists concerning adsorption 

 may be extended so as to include among 

 adsorbing surfaces those films which in 

 solutions surround bodies which are not 

 moistened by the solution. Physical in- 

 vestigations on this point are lacking. 



It is regarded by the authors as probable 

 that the insoluble substances used in ex- 

 periments here summarized act as adsorb- 

 ing surfaces for molecules or ions of the 

 poisonous substances used, the number of 

 molecules or ions thus adsorbed being, at 

 the extreme dilution here cited, sufficient to 

 remove a very considerable proportion of 

 the ions or molecules from the free solu- 

 tion. These are considered as collected 

 over the surfaces of the insoluble sub- 

 stances or films present in the solutions in 

 a layer molecularly much denser than the 

 free solution. In bringing about this redis- 

 tribution of molecules or ions, the free solu- 

 tion, when equilibrium has been established, 

 is necessarily much weakened in ions or 

 molecules. This affects the solution much 

 like the addition of water, bringing about a 



decreased number of molecules in a given 

 volume of the free solution. Hence the 

 close parallel seen between the conduct of 

 the radicles in solutions containing increas- 

 ing quantities of insoluble substances and 

 in progressively diluted aqueous solutions. 



Concerning the relation of the poisonous 

 substance to the adsorbing surfaces, a fur- 

 ther point is developed. In the solutions 

 of the heavy metals already referred to, 

 complete ionization is probable, and a pos- 

 sible electrical relation might be supposed 

 to exist. This is, however, not necessarily 

 so, since in solutions of thymol, a non- 

 electrolyte, a similar action was seen, indi- 

 cating that the electrical conditions in the 

 solution play no necessarily important part. 



The bearings of this work on practical 

 problems of plant physiology are very 

 many. It is impossible to argue from the 

 behavior of plants in water cultures to their 

 behavior in soil, since in the soil adsorption 

 is seen in a high degree of efficiency, where 

 films and Insoluble substances bring about 

 a situation entirely different from that seen 

 in a free solution. Hygroscopic water, so 

 called in works on plant physiology, is 

 doubtless to be regarded as an adsorbed 

 solution governed by the laws of adsorp- 

 tion. The recognition of this relation 

 would render physiological discussions 

 bearing on soil conditions much clearer. 

 The stock experiments in elementary plant 

 physiology demonstrating that solutions 

 poured from clean sand come out weaker in 

 molecules than they go in, is to be explained 

 as an example of adsorption rather than 

 absorption by the soil. The importance of 

 adsorption physiologically with reference 

 to the root system, especially the root 

 hairs, is seen when one bears in mind the 

 fact that in the ground water those por- 

 tions of the solutions are richest in mole- 

 cules which are adjacent to insoluble sub- 

 stances or films. Many relations of a plant 

 to its substratum are affected by this con- 



