chap, xxii.] CAPILLARITY. 241 



retain its spherical form, but spreads itself over the 

 smooth glass surface. That is to say, the force of 

 adhesion between the molecules of the glass and those 

 of the liquid has overcome the force of cohesion 

 between the molecules of the liquid. A drop of mer- 

 cury, however, will not lose its spherical form on 

 being placed on a perfectly clean glass surface. 

 That is to say, the cohesive force exerted between 

 the molecules of the mercury is able to overcome 

 the attractive action exerted by the molecules of the 

 glass on those of the mercury. Other liquids exhibit 

 similar phenomena, some adhering, others not adhering, 

 so that the degree of adhesive force varies with the 

 liquid. This is put in simpler language when it is 

 said that the water wets the glass, but the mercury 

 does not. But, while w T ater wets glass, it will not wet 

 some other substances. Thus, if the glass were greasy, 



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the drop of water would retain its spherical form, and 

 would readily roll off the plate. And, again, while 

 mercury does not wet glass, it will adhere to copper ; 

 so that the degree of adhesive force depends both on 

 the nature of the liquid and of the solid body with 

 which the liquid is in contact. 



Capillarity. These facts are held as affording 

 the explanation of capillary action. If a glass tube 

 of narrow bore be plunged vertically into a vessel of 

 water, the water will rise in the capillary tube above 

 the level of the surface of the water in the vessel. The 

 surface of the water in the tube will not be horizontal, 

 but will present what is called the CONCAVE MENISCUS 

 (yUTjz/t'o-Kos, a crescent). The surface, that is, will be 

 curved, a depression existing in the centre, and the 

 water rising where it is in contact with the walls of 

 the tube. This fact of the ascension of water in a very 

 narrow tube was noted at the commencement of the 

 seventeenth century by an Italian physicist. It Avas 

 supposed for a time to be due to the action of the 

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