REPORT ON CAPILLARY ATTRACTION. 269 



the facts whose explanations according to that theory have al- 

 ready been exhibited, and his mode of accounting for them dif- 

 fers not in any essential respect from that of Laplace, but chiefly 

 in a scrupulous avoidance of the use of mathematical symbols. 

 It will therefore be unnecessary to adduce the explanations of 

 any of these facts given by Dr. Young ; we will only advert to 

 some applications contained in his treatise which do not occur 

 in the other. 



Having first considered the rise of water in capillary tubes, 

 he proceeds to find the weight of water raised by the horizontal 

 surface of a solid elevated from the horizontal surface of a fluid, 

 and to determine the relation between the height of ascent in a 

 given tube to the height of adhesion ; that is, the height of ele- 

 vation above the ordinary level just when the fluid detaches it- 

 self from the horizontal surface of the solid. The fluid is 

 supposed to wet the solid. Hence as the fluid surface, being 

 horizontal where it is in contact with the solid, has no tendency 

 by its tension to depress, the weight of water raised is very 

 nearly equal to the hydrostatic pressure of a column of water 

 standing on the raising surface and equal in height to the height 

 of adhesion. This pressure he finds to be 50^ grains on a square 

 inch, agreeing very nearly with the result of experiments by 

 Taylor. If the raising surface be small, for instance a disc of 

 an inch in diameter, the curvature of the horizontal sections of 

 the raised fluid, which are convex outwards, will have a con- 

 trary effect to the curvature of the vertical sections which are 

 concave, and will consequently diminish the weight of fluid 

 raised. This also is confirmed by experiment. " The height 

 of ascent in a tube of given bore varies in the duplicate ratio of 

 the height of adhesion." 



The depression of mercury in capillary tubes is next considered, 

 and the author does not confine himself to the case in which the 

 surface of the mercury is spherical, which is true only when the 

 diameter of the tube is very small. In tubes less than half an 

 inch in diameter, the surface is very nearly that of an oblate 

 spheroid. The depressions for different tubes calculated theo- 

 retically, are compared with a table of experiments made by 

 Lord Charles Cavendish to ascertain the depression of mercury 

 in different barometer tubes. 



The height of adhesion of mercury to glass, and to substances 

 which it is capable of wetting, such as gold, silver, tin, &c., as 

 well as the thickness at which a portion of mercury will spread 

 out on glass and on substances wholly incapable of attracting it, 

 are quantities all determinable by this theory, and being cal- 



