408 



CAPILLARY ATTRACTION. 



Capillary Exp. 21. When the column of water is thus made 

 Attraction. to move a l on g t h e tube, it seems to suffer a resistance 

 V ""-Y" 1< -' as i t approaches to either end, and it does not com- 

 pletely reach the extremity of the tube till the tube 

 is almost inverted. 



Exp. 22. When a capillary tube is immersed in 

 mercury, or in any metal in a state of fusion, the 

 fluid, instead of rising, is depressed in the tube be- 

 Pi.4TE CX. low the general level. See Fig. 4. Gellert found that 

 Fig. * when a glass tube was immersed in melted lead, the 

 depression multiplied by the bore was 0.054. Messrs 

 Hauy and Tremery found, that the depression of mer- 

 cury in a capillary tube one thousandth part of a metre 

 in diameter, or 0.039371 of an English inch, was 

 0.2887 of an English inch, the product being .01 137. 

 The ultimate product inferred from Lord Charles 

 Cavendish's experiments is .015. 



Exp. 23. If a drop of mercury be introduced into 

 a conical capillary tube, held in a horizontal position, 

 the mercury will move towards the wide end. 



Exp. 24?. By observing the surface of a column 

 of mercury depressed in a capillary tube, or inclosed 

 in a barometer tube, it will be found to be convex 

 Upwards. Mr Hauy has endeavoured to show, that 

 this convexity differs very little from the form of a 

 hemisphere. Dr T. Young maintains that this result 

 is inaccurate, and that the angle formed by the sur- 

 face of the mercury with the side of the tube is 

 140. He obtained this result, by observing in what 

 position the light reflected from the mercurial sur- 

 face began to reach the eye, and he has found it cor- 

 rect, from the comparison of a great variety of ex- 

 periments of different kinds. This ingenious phi- 

 losopher has prosecuted this branch of capillary 

 attraction with great ability and success. He has 

 calculated the precise form of the surface of the mer- 

 cury in a variety of cases. In order to confirm these 

 calculations, he employed another method, which 

 consists in finding the mass of the quantity of fluid, 

 supported by the tension of the surface at each con- 

 centric circle, and inferring from this the inclination 

 of the curve to the horizon, assuming for the mean 

 * height the height of the external circumference of 



each portion ; a supposition which almost compen- 

 sates for the omission of the curvature of its surface. 

 As a specimen of these methods, we shall insert the 

 following Tables, by means of which the curves may 

 be correctly delineated. They are suited to a cen- 

 tral depression of 0.007. 



First Method by the Curvature. 



Second method by the Tension. 



Capillary 



Attraction. 



Dr Young has embodied the results of these calcu- 

 lations in the following formula, which gives the 

 central depression without any perceptible error, 



0.015 d . , . f , 



e-=. , , . . , which is nearly half the versed sine 

 aa-j-O.lo 



of a spherical surface, and theny == -I-, | e 



CL 



14.5 e 3 . This approximate formula supposes the 

 surface to be spherical, and is corrected by a com- 

 parison with the results of the calculations, so as t 

 agree with them all without an error of one two- 

 thousandth of an inch in the most unfavourable cases. 

 Dr Young has also found a formula, when the dia- 

 meter of the tube is moderate, for shewing the dif- 

 ference between the central and marginal depression, 

 which may be of the greatest service in correcting 

 the height of the barometer, whether we have ob- 

 tained a measure of the highest or lowest point of 

 the surface 



_5 d -f 

 S ~ 



15(5d+ lOOrfs) 4. 18 

 If d were very large, it would require some farther 

 correction, g being ultimately too great by 0.0069. 

 As our limits will not permit us to pursue this in- 

 teresting subject any farther, we must refer our 

 readers to Dr Young's able paper on the Cohe- 

 sion of Fluids, published in the Phil. Trans, for 

 1805, and in his Lectures on Nat. Phil* vol. ii. p. 

 666669. 



Exp. 25. When the mercury and the capillary 

 tube are perfectly dry,' the fluid will rise above the 

 general level, like all other liquids. This was ascer- 

 tained by Professor Casbois of Metz, who boiled the 

 mercury several times, in order to free it from all hu- 

 midity, and expel any foreign particles. By drying 

 the mercury and the tube to a very great degree, 

 Messrs La Place and Lavoisier constructed barome- 

 ters, in which the mercurial column was terminated 

 above by a plane surface, and they even succeeded in 

 rendering the upper surface of the mercury concave. 

 The observations given under Experiment 24 are re- 

 ferable to barometers constructed in the usual way. 



Exp. 26. If two plates of glass be placed parallel Ascent 0f 

 to each other, at the distance of about T ^y of an fl u j,] s be _ 

 inch, the water in which they are immersed will rise tween 

 one inch above its level in the vessel ; and when the plates f 

 plates are placed at different distances, the heights to 8 la$$ 

 which the water will rise, will be reciprocally pro- 

 portional to the distances of the plates. 



Exp. 27. If a capillary tube be taken of such a 

 magnitude, that the diameter of its bore is equal to 



