SURFACES OF DISCONTINUITY 677 



the liquid attracts the surface as strongly as (or indeed more 

 strongly than) itself, the contact angle is zero. On the other 

 hand, an obtuse angle of contact, such as in the case of mercury 

 and gl ass, indicates relatively small adhesion or absence of wet- 

 ting. Reference should also be made to the "heat of wetting" 

 in this connection. Heat generally results from the making of a 

 contact between the surfaces of a liquid and a solid. This 

 heat is the total energy of the wetting of the solid by the liquid, 

 and is connected with the adhesion or free energy of wetting 

 by the same relation as exists between the total and free energies 

 of a surface, as can be easily shown by combining the three 

 equations derived thus for the three interfaces, solid-air, liquid- 

 air, solid-liquid, with the definition of adhesion given above. 

 In fact if WsL is the work of adhesion, the expression for the 

 heat of wetting per unit area is 



dWsL 



However, there seems to be considerable difficulty involved in 

 calorimetric determinations of the heat of wetting, as widely 

 divergent results are obtained by different experimenters, 

 although the existence of the phenomenon has been known for 

 over a hundred years. In consequence, the result just quoted 

 has not been verified, since it would require, in addition to a 

 knowledge of the changes of aLA and a with temperature (which 

 could be obtained with sufficient precision), reliable values of 

 the heat of wetting, which appear to be wanting. The reader 

 should consult Adam's Physics and Chemistry of Surfaces and 

 Rideal's Introduction to Surface Chemistry, Chapter V, for fur- 

 ther information and references. The matters just dealt with 

 are also closely connected with the question of the conditions 

 under which a liquid will spread as a film over a solid, or remain 

 in compact form as a drop. For an adequate treatment of this 

 important point and its bearing on lubrication reference can be 

 made to Chapter VII of Adam's book, as space is not available 

 for more than a passing remark here. In the same volume a 

 brief account is given of the connection between contact angles 



