Spread of Liquids on Solid Bodies. 431 



therefore, be surprising that the electric discharges meeting a 

 surface of glass in greater or less intensity from the elevated 

 or depressed parts of a coin, remove the adherent liquid or 

 gaseous film more or less strongly. Add to this the heating 

 action of the discharges passing across the film of air between 

 the coin and the glass. The elevated and depressed parts 

 must when again breathed on be distinguished in the breath- 

 figure as places of greater or lesser edge-angle. 



Still more readily do the electric breath-figures investigated 

 by Gr. Karsten * and Biess | succeed, formed upon the cleanest 

 possible surface of a recently melted plate of pitch. 



These electrical breath-figures can be exhibited with vapour 

 of mercury or of iodine J, instead of the moisture of the breath, 

 since these must likewise form small lenticular drops of vary- 

 ing edge-angle. 



If an electric spark has been discharged upon the surface 

 of a plate of glass, mica, or*metal, the adherent film will have 

 been as good as completely removed from the places touched 

 by the discharge. Then the water condensed on breathing 

 will form lenticular drops no more, but will spread in a con- 

 tinuous film : the place hit by the spark appears glossy upon 

 a dull ground and forms the electric breath-figure described 

 by Eiess. 



I think the results of the foregoing investigations may be 

 summed up as follows : — 



1. The long-known properties of the bounding surface 

 common to two fluids may be extended to the bounding sur- 

 face common to a fluid and a solid body. 



2. The common bounding surface of a solid body 1, and of 

 a liquid 2, tends to become a minimum, or, as we may say, it 

 is governed by a surface-tension « 12 independent of the 

 geometrical form of the surface, and dependent only upon the 

 nature of the two substances 1 and 2. 



3. The magnitude of the edge-angle of a solid body 1 and 

 a liquid 2, which are both bounded by fluid 3, is determined 

 only by the nature of the three substances, and is independent 

 of the geometrical form of the surface. 



4. The principal law of the capillary theory, given by Dr. 

 Thomas Young, concerning the constancy of the edge-angle 

 of the free surface of a solid body and of a liquid is a special 

 case of the law expressed in summary 3, in which fluid 3 

 consists of air. 



* Fogg. Ann. lvii. p. 493 (1842). 

 t Riess, Reibunqselecti'icitat, ii. p. 224. 

 - % Karsten, Pogg\ Ann. lvii. p. 496 (1842). 



