FOURTH REPORT — 1834. 



elevation in glass tubes, probably by reason of different degrees 

 of humidity of the interior of the tubes. When the tubes were 

 well wetted, Gay-Lussac found the elevations to be always very 

 nearly the same in different experiments. As the weights re- 

 quired to detach discs from fluid surfaces can be measured with 

 considerable precision, the accordance of the preceding experi- 

 mental and theoretical values serves to verify the experimental 



values of — . 

 a. 



Equal discs of different substances perfectly wetted by a fluid, 

 ought to raise columns of the same weight, because the resist- 

 ance to the separation of the disc is, in each case, produced by 

 the adhesion of the fluid to itself, that is, to the stratum of fluid 

 that lines the inferior surface of the disc. 



As the angle of contact of mercury with glass under water is 

 nearly 180°, a glass disc applied under water to the surface of 

 mercury would not be capable of drawing up any portion of 

 mercury on being raised from the surface. 



These two conclusions from the theory have been confirmed 

 experimentally by Gay-Lussac. 



(3.) The analytical calculation for determining the form of a 

 large drop of mercury serves also to find the depression of this 

 fluid in a glass tube of large diameter. Gay-Lussac ascertained 

 by experiment that the height at which a large drop of mercury 

 stands on a horizontal plane of glass is 3™'"378 (= '133 of 

 an inch), agreeing very nearly with the result of a like experi- 

 ment by Segner, The drop was circular and one decimetre, or 

 3*937 inches in diameter, and the temperature at 12°'8 of the 

 centigrade thermometer. To calculate the height theoretically, 



. . ^ 2 



it is necessary to know the value of — for mercury in a glass 



tube, and its angle of contact with glass. Laplace takes for 

 the former 13 mi-mi^ (_ -0201 square inch), and for the latter, 

 152° ( = 136°*8). These values, he says, are mean results ob- 

 tained by comparing several observed capillary phaenomena with 

 the theory, and may be further rectified by more numerous ex- 

 periments. The height of a large drop of mercury given by the 

 theory by means of these data is 3"''-39664. 



brace a great variety of fluids ; but the tube does not appear to have been 

 moistened in its whole extent. Tlie results are tabulated in the article above 

 refeiTed to, as well as those from the experiments of Mr. B. Martin, which, for 

 the same fluids, uniformly give a higher value of the constant. The values of 

 the constant for alcohol and oil of tui-pentine, as found by Gay-Lussac, do not 

 differ materially from the determinations of others. Sir David Brewster finds for 

 alcohol -0178, for oil of turpentine -0187; Martin, for the same fluids, '018 

 »nd '022; Musschenbroek for alcohol -021, 



