CAPILLARY ACTIVE SUBSTANCES 51 



mercury in the bottom of the vessel holding the acid. The position 

 of the meniscus in the capillary is observed through a microscope 

 or is projected on a screen. A very slight increase in the charge 

 on the mercury-acid interface will lead to a movement of the 

 mercury in the direction of the current. That is, if the mercury 

 holds the positive lead the meniscus will fall (S.T. lowered) ; con- 

 versely, if the negative pole from the battery is attached to the 

 capillary, the meniscus will rise (S.T. raised). The extent of move- 

 ment depends on the difference of potential developed (Figs. 38 and 43). 



B. Solutions. — The surface layers of electrons may be increased 

 or decreased by the addition of solutes which, by altering the 

 intrinsic energy of the solution, cause a redistribution of energy, 

 and so new surface relationships are produced. The material 

 dissolved in a fluid is not dispersed regularly, but is generally found 

 more concentrated at surfaces. According to the Gibbs-Thomson 

 principle, those solutes which tend to lower surface tension are 

 found at the surface, while those which raise surface tension are 

 least concentrated at the surface, i.e., " The concentration throughout 

 a fluid tends to be so adjusted as to reduce the energy at every p)oint 

 in it to a minimum.'''' Very few substances raise surface tension, 

 and that to a very slight extent. Strongly dissociated inorganic 

 salts have very little effect either way, but are usually found 

 adsorbed to surfaces. This fact is attributed not to any property 

 of the salts, but to the unsatisfied valencies existing at any surface 

 wetted by the solvent. (See also Adsorption, p. 53.) 



C. Capillary Active Substances. — The surface tension of water is 

 markedly diminished by certain organic substances with long 

 carbon chains. The longer the carbon chain, and the smaller the 

 number of decidedly electro-jjositive and (particularly) electro-nega- 

 tive groups (such as — OH and — COOH) they possess, the more 

 powerful is their action. These substances, which are also distin- 

 guished by a high degree of adsorbability (q.v.) and by poW'Crful 

 biological actions (cf. anaesthetics, etc.), are called capillary active 

 from their effect in lowering the level of water in a capillary tube. 

 They have also a low surface tension themselves and are of interest 

 because of the orientation of their molecules on the surface of water. 

 Typical physiological capillary active substances are found in 

 saliva, bile, blood and milk. These substances all have in their 

 carbon chain a radicle which is particularly soluble in w^ater, e.g., 

 carboxyl (COOH), hydroxyl (OH), COOCH3 or CN. Apart from 

 this polar radicle, the remainder of the molecule is insoluble or 

 markedly less soluble in the solvent. They are, therefore, arranged 

 like a fisherman's floats with their soluble polar ends in the water 

 and the rest of the molecule standing vertically out of the water 



4—2 



