SURFACE ACTION 69 



7. The rate of action of enzymes is controlled by adsorption, but full discussion 

 will be more conveniently deferred until later. 



8. When the protoplasmic contents of a ciliate infusorian or the root hair 

 of a plant are pressed out into water, a membrane is at once formed on the free 

 surface of the protoplasm. This fact has been described by Kuhne (1864, p. 39) 

 and by Pfeffer (1897, i. pp. 92, 93). The nature of this membrane will be 

 discussed in Chapter V., and it will suffice to call attention to it here as being 

 undoubtedly due to surface concentration of cell-constituents which lower surface 

 energy. 



9. The blue substance formed by the action of iodine on starch has long been 

 familiar, but its nature as an adsorption compound has only recently (1912) 

 been made clear by the work of Barger and Field (1912). They also show that 

 similar blue compounds are formed by substances of very varied chemical nature, 

 such as saponarin, cholalic acid, and lanthanum acetate. 



10. That powerful action on cell processes can be exerted by substances 

 which do not penetrate beyond the surface of the cell is shown by a very 

 interesting experiment of Warburg (1910, pp. 310, 311, 313). The oxygen 

 consumption of the fertilised eggs of a sea-urchin in an artificial sea-water is 

 doubled by the addition of 10 c.c. of decinormal sodium hydroxide to 1 litre 

 of the sea-water, the development being, at the same time, stopped. If the 

 cells are stained previously with neutral red, which does not affect their develop- 

 ment, no change of colour takes place on addition of sodium hydroxide ; whereas 

 with ammonia, to which the cell membrane is permeable, the cells become yellow 

 in less than one minute. Athough uninjured by the concentration of ammonia 

 used, the oxygen consumption is only increased by 10 per cent, instead of the 

 100 per cent, when the H* ion concentration is changed only at the surface. 



THE CONDITION OF ADSORBED MATERIAL 



There is one point that it is of some importance to understand clearly. When 

 an electrolyte, say acetic acid, is adsorbed by charcoal, it is fixed for the time on 

 the surface. By this statement it is not meant to imply that the same identical 

 molecules remain in the same place, but that a certain proportion of the acid is 

 taken out of solution and cannot take part in such properties as the electrical 

 conductivity or the osmotic pressure of the system. A mixture of acetic acid and 

 charcoal has the electrical conductivity of the liquid phase alone. Similarly, when 

 the particles of the adsorbent are too large to give an osmotic pressure (see 

 Chapter V.), the osmotic pressure of the system is due only to the solution. The 

 adsorption compound of charcoal plus acetic acid, or other adsorbed electrolyte, 

 has no higher osmotic pressure nor conductivity than the charcoal itself. Some 

 observers are inclined to attribute, incorrectly, the osmotic pressure undoubtedly 

 shown by certain colloidal solutions to adsorbed electrolytes or crystalloids. 



In the state of adsorption, salts, not being electrolytically dissociated, give 

 none of their characteristic reactions. Iron, for example, is in what is sometimes 

 called a "masked" condition. 



Ruer (1905) found that when chlorides are adsorbed by colloidal zirconium hydroxide, no 

 reaction with silver nitrate is given. The presence of chlorine in colloidal ferric hydroxide can 

 only be detected by transforming the colloidal solution into a true solution by means of nitric 

 acid, that is, by abolition of the adsorbing surface. 



On the other hand, it must not be forgotten that adsorbed substances are only 

 fixed as long as the solution with which they are in equilibrium remains of the 

 same concentration, which may, however, be very low. Nevertheless, by repeated 

 washing, practically the whole of the adsorbed matter may be removed, although 

 an infinite number of changes of water is theoretically necessary. If charcoal 

 which has adsorbed sugar be placed inside an osmometer, whose membrane is 

 permeable to water and sugar, but not to charcoal, sugar will pass out to water 

 on the outside, and by repeated changes of this water the sugar can be almost 

 entirely removed from the charcoal inside. Substances merely adsorbed cannot 

 be prevented from escape to water ; in order that they shall not do so, they must 



