328 ORIGIN AND DEVELOPMENT OF OBE-DEPOSITS. 



this surface tension, while a larger quantity spreads itself out 

 under the influence of gravity until it appears to be a perfect 

 plane. The gravity is acting all the time, but in the case of the 

 small mass with the comparatively large surface the surface 

 tension to all appearance overcomes the action of gravity. 



A closely related case is that of liquids in communicating 

 vessels, or in two fissures connected by a transverse fissure, 

 when under ordinary circumstances the water will stand at the 

 same level in both. If, however, one of the vessels or fissures 

 is very narrow, say x^o^l^ ^^ ^^ inch or less, the liquid wiU stand 

 very notably higher in this than in the wider one, thus apparently 

 setting the force of gravity at defiance ; so that, given sufiiciently 

 small or narrow apertures, the tension becomes so great that 

 solutions have power to penetrate porous substances against 

 strong positive steam or air pressures, equal to many pounds on 

 the square inch, as shown by Daubree and others. 



Such are what may be called the mechanical effects of surface 

 tension ; there are equally remarkable chemical effects. Many 

 decompositions are effected, and many precipitations are induced 

 by it, as was long ago demonstrated by Becquerel, Hunt, and 

 others.'^' Among natural examples of the operations of this force, 

 I may refer to the thin plates of native copper found in the 

 joints of el van and killas at the Grwennap Mines and many other 

 places, and the arboreal markings of manganese which are so 

 common in the finer joints of rocks almost everywhere. f 



One of the effects of this penetration of solutions through 

 narrow fissures, and of the deposition of foreign matter within 

 them must be to widen them, so that there is a constant tendency 

 for potential structural planes to be converted into actual, and 

 for actual divisional planes to become wider. Of course this 



* See experimeuts of Becquerel quoted by Hunt, and also his own, in his 

 "Contribution, &c.," Trans. R.G.S.O., ix, 23. 



f It has been shown by the experiments of Dr. Hofmann and Mr. Witt that 

 a certain portion of the salts dissolved in water are separated by passing through 

 the filter-beds of the London Water Companies. Mr. E. Hunt, in his Economic 

 Geology of Devon and Cornwall (Bath and West of England Agric. Journ., xvi, 

 1868) quotes this, and also refers to certain experiments of Spencer, Normanby, 

 and Graham ; and adds "The papers referred to clearly indicate the operation of 

 a force which is most active in all the works of nature." 



