September 16, 1922] 



NA TURE 



177 



Letters to the Editor. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of NATURE. No notice is 

 taken of anonymous communications.'] 



Capillarity. 



In Mr. W. B. Hardy's excellent " Historical Notes 

 upon Surface Energy and Forces of Short Range " in 

 Nature of March 23, vol. 109, p. 375, he remarks 

 that " the exact way in which the attractive forces 

 act in causing the rise of fluid in capillary tubes 

 and the spreading of fluid over solid and fluid sur- 

 faces is still obscure." He evidently rejects all 

 explanations by any Laplacean conception of molec- 

 ular attraction. He probably holds that the ex- 

 planation is to be sought in the modern electric 

 theory of the constitution of matter, but that this 

 theory has not as yet been developed far enough to 

 throw sufficient light on the question. By the use 

 of the term " attractive," however, he restricts the 

 inquiry to a limited class of forces in terms of which 

 these phenomena are to be explained. It is just 

 possible that this restriction may preclude the solution 

 of the problem. 



That it is possible to conceive of a force which 

 cannot be put in this class of " attractive " forces of 

 short range may be shown, if a mass consisting 

 of a single molecule exists at all, it has a position in 

 ether-space at some particular instant. It may be 

 considered isolated from all other masses. There is, 

 therefore, a closed boundary within which there are 

 properties differing from those not within this 

 boundary. Any part of this closed boundary may 

 be conceived as an area between space-regions 

 called " mass " and " no-mass." If we say that 

 this enveloping area tends to become less, we have 

 defined a " force " which cannot be included in the 

 class of attractive forces. Further, if we say that, 

 when two such enclosures come into contact, the tendency 

 to decrease does not exist in the area of contact, since 

 there is no distinction of properties on opposite sides 

 of this area, we have defined the law of the " force " 

 for like masses. Finally, if we say that when the 

 two masses are unlike there is this distinction of pro- 

 perties and, consequently, a tendency of the common 

 area to decrease, we have extended the law to unlike 

 masses. Space will not permit the elaboration of 

 these conceptions. 



The question is : Does this " force " as conceived 

 above reallv exist ? Whether it does or not, repeated 

 applications of the law as stated will account for 

 surface tension of visible liquid and solid surfaces in 

 terms of it. It will be admitted that the phenomenon 

 of coalescence of visible spheres of like liquids is a 

 direct application of the law. Also, in his letter on 

 " Cohesion " in Nature of January 5 (vol. 109, p. 10), 

 the present writer has shown experimentally that 

 visible exterior cohesion and adhesion in solids and 

 liquids may be accounted for by surface tension forces 

 alone, and, consequently, may be explained by 

 molecular (surface) tension. Now these and even 

 eapiUary-rise and fluid-spreading might conceivably 

 be explained by intermolecular electric forces, though 

 this has not yet been done ; but it is difficult to see 

 how the latent heat of a gas can be so accounted for, 

 since in a gas the molecules are too widely separated 

 to admit of short range intermolecular action at all, 

 and the condensation of a gas, by which alone this 

 great amount of heat is made available, is unattended 



NO. 2759, VOL. I IO] 



by any chemical change or electrical effect. This 

 latent heat, however, may be fully accounted for by 

 molecular surface tension, as the writer has shown 

 in the Phil. Mag. v. 41, p. 877. 



Either, then, the latent heat of a gas must be 

 otherwise accounted for, or the existence of molecular 

 surface tension must be admitted. This in turn, 

 it is true, may eventually be explained by the action 

 of electric forces in the interior of the isolated molecule. 

 In the meantime the writer offers the following ex- 

 planation of capillary-rise and surface-spreading in 

 terms of molecular tension as conceived above. 



The following phenomena are illustrative, (a) If 

 two free spheres of immiscible liquids be brought into 

 contact, the mass of the one suddenly proceeds 

 completely to envelop the mass of the other. (6) If 

 a free liquid sphere and a small solid which does not 

 dissolve in the liquid be brought into contact, one of 

 two actions takes place : either the mass of the liquid 

 forms a closed sheath about the solid, or the mass of 

 the solid tends to envelop the liquid, but is prevented 

 by internal cohesion from assuming the necessary 

 form. The liquid then stands out as a curved mound 

 on the solid with a distinct "capillary" angle. 

 With water mercury acts as a solid metal. It is on 

 this differential action of this surface force of water 



F 





Rq.3. 



Fiq.3 



in union with those of small particles of minerals and 

 rocks that the flotation processes for mineral separa- 

 tion depend. 



When a liquid is in contact with a solid, there are 

 three areas in which this tendency to decrease exists 

 in different degrees, namely, the liquid-air, the solid- 

 air, and the solid-liquid areas, the sum of the latter 

 two areas being constant. At contact the only 

 possible way in which these latter two tendencies 

 can result in action is to decrease the solid-air area 

 from its maximum by increasing the solid-liquid area 

 from zero. Experiments show (Nature, ibid.) that 

 this change always proceeds to some extent. In the 

 case of water and rock it proceeds until all the solid- 

 air area becomes solid-liquid area. It does not reach 

 this limit in the case of water and a mineral. 



But the extent of this fluid-spreading depends 

 further on (1) the tendency to decrease of the liquid-air 

 area, unrestricted by internal cohesion, (2) the force 

 of gravity acting on the liquid mass, (3) the form of 

 the solid area, and (4) the amount of the liquid. 

 Thus, a small drop of an oil such as oleic acid may 

 not spread far on a horizontal glass surface, since the 

 decrease in the glass-air area involves increases in 

 both glass-oil and oil-air areas ; whereas in capillary- 

 rise this decrease does not involve any increase in 

 oil-air area. The oil will, therefore, ascend until its 

 increasing weight balances the tendency to decrease 

 (tension) in the glass-air area within the tube. Again, 

 a hanging drop of the oil will completely envelop 

 a large fragment of glass and hold it against its 

 weight. 



It is plain, then, that methods of measuring surface 

 tensions of liquid-air and liquid-liquid areas should 



