SURFACE ACTION 67 



gum, which acts as a protective colloid, ensuring fine subdivision of the 

 manganese, in the way to be described in the next chapter, is thrown down by 

 alcohol, it carries with it, in a state of adsorption, the manganese. 



The inorganic salts, usually associated with proteins, are probably adsorbed. 

 The law expressing the way in which they are removed by water shows that 

 they are not merely admixed, while the fact that they are so removed shows 

 that chemical combination is not in question (see my investigation of gelatine, 

 Bayliss, 1906, pp. 179-185). 



Several other compounds in which adsorption plays a part will be discussed 

 in later pages. 



It appears to be held by some observers that many of these adsorption com- 

 pounds, especially those in which lecithin occurs, are more of the nature of solid 

 solutions. The ratio in which their constituents stand to their concentration in 

 the reacting mixture points rather to surface condensation, although solid solution 

 cannot be entirely excluded. Loewe (1912, pp. 216-218) finds that the substances 

 known as "lipoids," of which lecithin is an example, take up dyes, hypnotics, 

 and tetanus toxin in a way which is not compatible with the solid solution views 

 but with an adsorption process. The exponents of the equations, expressing the 

 relation of the amount taken up to the concentration of the solutions, are not 

 of such values as to admit of the interpretation of distribution between phases 

 in unequal proportion. Moreover, when nicotine or methylene blue in solution is 

 allowed to remain for a long time in contact with lipoid matter, no diffusion is 

 found to take place into the interior of the lipoid. It appears, therefore, that the 

 action is a surface one. 



ADSORPTION AS CONTROLLING CHEMICAL ACTION 



That adsorption does not preclude subsequent true chemical combination is 

 obvious. So far is this the case that, in many cases, chemical change seems to 

 necessitate preliminary adsorption. One at least of the constituents of an 

 adsorption compound possesses, of course, a surface, either the visible one of such 

 materials as paper, cell granules, and various fabrics or tissues, or the ultra- 

 microscopic surfaces of colloidal particles. Substances in such states of aggregation 

 are naturally inert, as far as chemical activity is concerned, so that when the 

 chemical reaction is between the components of the phase possessing the surface 

 and the adsorbed substance, it is to be expected that it will proceed very slowly. 



0. C. M. Davis (1907) finds that charcoal takes up iodine with great rapidity up to a 

 certain point of apparent equilibrium, but that, if the components are allowed to remain 

 together for a longer time, a very slow further disappearance of iodine goes on. The first part 

 of the process differs, as would be expected, according to the particular kind of charcoal used, 

 since the surfaces would vary. The second process is the same for various kinds of charcoal 

 and is interpreted by Davis himself as being a passage of iodine into the mass of the solid ; a 

 solid solution, in fact. It is suggested by Freundlich (1909, p. 173) that chemical combination 

 is more probable, since iodine is a very reactive substance. This suggestion explains why the 

 second part of the process is irreversible and does not vary with the kind of charcoal used. 



It is probable that the fixing of dyes on tissues by heat is due to chemical 

 combination. When Congo-red is taken up by filter paper in the absence of 

 electrolytes, it is readily washed out again. But if raised to 100 it becomes 

 fixed. The same process goes on slowly at ordinary temperatures. 



There are two classes of reactions in which the rate of chemical combination 

 is controlled by adsorption. The first is when the two reacting substances are 

 condensed on the surface of a third and combine together there, leaving the 

 adsorbing surface in the end unaltered. This process is one of those that we 

 shall learn later to call " catalytic." Examples of such reactions are : 



(1) The production of sulphuric acid under the influence of platinum, in which 

 it has been shown by Bodenstein and Fink (1907) that the rate of the reaction 

 is governed by the adsorption of SO 3 on the surface of the platinum. 



(2) The effect of platinum on the reduction of titanic sulphate by hydrogen 

 (Denham, 1910). 



(3) The decomposition of ozone by heat takes place on the walls of the 

 containing vessel, or other surface present (Perman and Greaves, 1908). 



