Potential Energy of Liquid Surfaces. 453 



phenomena carefully, reserving to myself to describe them in 

 a more developed investigation. 



II. If the increment of surface <iS is produced by a solid 

 body immersed in the mass m and not wetted by the liquid, a 

 work TjdS must be developed, T x designating the potential 

 energy of the unit of surface of contact. Now I have de- 

 monstrated * that this energy constitutes a real tension ; it 

 follows that in this case, again, the liquid mass will undergo 

 a diminution of temperature which will be greater in propor- 

 tion as the mass m and the specific heat k are less. 



I shall shortly endeavour to prove this assertion by direct 

 experiments ; but, as I shall show further on, there are facts 

 which demonstrate it indirectly. 



III. Suppose in the third place, that the increment dS 

 given to the surface is produced by a solid body plunged in 

 the mass m and wetted by the liquid ; then the potential 

 energy T 2 of the unit of surface of contact of the solid and 

 liquid is not due to a force of tension, but to a force of ex- 

 tension, as I have sought to establish in the Note above cited. 

 It hence follows that the potential energy of the mass, instead 

 of being increased by the augmentation of the surface, is, on 

 the contrary, diminished; and consequently this loss of po- 

 tential energy must be compensated by a heating of the mass 



dT 



on. This follows immediately from formula (1), where -=- is 



then positive, and consequently also dQ. 



This curious theoretical consequence seems fully verified by 

 the numerous experiments made in 1822 by Pouilletf, who 

 ascertained that heat is disengaged whenever a solid, pre- 

 viously reduced to powder or filings as fine as possible and 

 then perfectly dried, is wetted with any liquid : he found, by 

 delicate measurements, that glass becomes heated £° Centi- 

 grade when wetted either with water, oil, alcohol, or acetic 

 ether ; under the same treatment porcelain receives a heating 

 of about J°; lastly, clay becomes nearly 1° warmer when 

 soaked with water, and J° with acetic ether. 



Formula (1) indicates that the more considerable the in- 

 crement of the surface, of course up to a certain limit, the 

 more energetic will be the disengagement of heat ; now this, 

 again, was verified by the experiments of Pouillet. Among 

 the inorganic bodies which he tried, it was the most porous, 

 such as clay (brick), which became the most heated ; but 



* " Sur les proprietes de la surface de contact d'un solide et d'un liquide," 

 Bull, de V Acad. Roy ale de Belgique, tome xl. p. 341. 



t " Meinoire sur de nouveaux plieuomenes de production de chaleur/' 

 Ann. de Chimie et de Physique, 1822, tome xx. p. 141. 



