SBCT. XV. CAPILLARY ATTRACTION. Ill 



The attractions and repulsions arising from capillarity 

 present many curious phenomena. If two plates of 

 glass or metal, both of which are either dry or wet, be 

 partly immersed in a liquid parallel to one another, the 

 liquid will be raised or depressed close to their surfaces, 

 but will maintain its level through the rest of the space 

 that separates them. At such a distance they neither 

 attract nor repel one another ; but the instant they are 

 brought so near as to make the level part of the liquid 

 disappear, and the two curved parts of it meet, the two 

 plates will rush toward each other and remain pressed 

 together (N. 172). If one of the surfaces be wet and 

 the other dry, they will repel one another when so near 

 as to have a curved surface of liquid between them ; but 

 if forced to approach a little nearer the repulsion will be 

 overcome, and they will attract each other as if they 

 were both wet or both dry. Two balls of pith or wood 

 floating in water, or two balls of tin floating in mercury, 

 attract one another as soon as they are so near that the 

 surface of the liquid is curved between them. Two 

 ships in the ocean may be brought into collision by this 

 principle. But two balls, one of which is wet and the 

 other dry, repel one another as soon as the liquid which 

 separates them is curved at its surface. A bit of tea 

 leaf is attracted by the edge of the cup if wet and re- 

 pelled when dry, provided it be not too far from the 

 edge and the cup moderately full ; if too full, the con- 

 trary takes place. It is probable that the rise of the 

 sap in vegetables is in some degree owing to capillarity. 



SECTION XV. 



Analysis of the Atmosphere Its Pressure Law of Decrease in Density- 

 Law of Decrease in Temperature Measurement of Heights by the 

 Barometer Extent of the Atmosphere Barometrical Variations Oscil- 

 lations Trade Winds Monsoons Rotation of Winds Laws of Hur- 

 ricanes Water-Spouts. 



THE atmosphere is not homogeneous. It appears 

 from analysis that of 100 parts 79 are azotic gas, and 21 

 oxygen, the great source of combustion and animal heat. 

 Besides these there are three or four parts of carh 



