470 



HYDRODYNAMICS. 



Cohesion of 

 Fluids. 



Labours of 

 La Place, 



1805. 



OnCpillary s ' lon O f Fluids, which appeared in the Philosophical Trans- 

 aniUhe 11 actions f r 1805 : and in which he has anticipated many 

 f V 16 views afterwards brought forward by La Place. 

 This paper is divided into 8 sections. Sect. I. Contains 

 the general principles; Sect. II. Treats of the form of 

 the surface of fluids; Sect. III. Contains the analysis 

 of the simplest form ; Sect. IV. Contains the application 

 of it to the elevation of particular fluids; Sect. V. 

 Treats of apparent attractions and repulsions; Sect. 

 VI. Treats of the physical foundation of the law of su- 

 perficial cohesion; Sect. VII. Of the cohesive attraction 

 of solids and fluids ; and Sect. VIII. is entitled, addi- 

 tional extracts from La Place, with remarks. A gene- 

 ral account of Dr Young's views, and some of his ex- 

 perimental results, will be found under our article on 

 CAPILLARY ATTRACTION, p. 408, 412. 



In December 1805, M. le Comte La Place laid before 

 the National Institute of France a Dissertation on Ca- 

 pillary Attraction, which is marked with the high genius 

 of its distinguished author. In 1806, it was published 

 under the title of Supplement au dixieme Lime du Trai- 

 le de Mecanique Celeste, sur I' Action Capillaire. In the 

 theory advanced by Clairaut, it was supposed that the 

 action of the capillary tube extended to a sensible dis- 

 tance, and even to the fluid particles in its axis ; and, 

 instead of attempting to discover the law of that action, 

 he contented himself with observing that there were 

 an infinity of laws of attraction, which, if substituted 

 in his formulae, would give results corresponding to 

 those obtained from experiment. M. La Place, how- 

 ever, was anxious to ascertain the precise law of at- 

 traction which represented the phenomena ; and, after 

 long researches, he at last succeeded in discovering, 

 that all the phenomena could be represented by the 

 same law which represents the phenomena of refrac- 

 tion, namely, the law in which the attraction is only 

 sensible at insensible distances ; and upon this he has 

 founded a rigorous theory of capillary attraction. The 

 first Section of this work treats of the Theory of Capil- 

 lary Attraction ; and the second contains its applica- 

 tion to some of the experiments of Hawksbee, and to 

 others made at La Place's request by MM. Hauy and 

 Tremery. In the year 1807, La Place published his 

 Supplement a la Theorie de V Action Capillaire ; the ob- 

 ject of which was to perfect the theory which he had 

 given to extend its application to confirm it by new 

 comparisons with experimental results and, in pre- 

 senting under a new point of view the effects of capil- 

 lary action, to shew the identity of the attractive force 

 upon which it depends with those which produce che- 

 mical affinities. 



This Supplement treats, 1. Of the fundamental Equa- 

 tion of the Theory of Capillary Action. 2. Of a new 

 manner of considering Capillary Attraction. 3. Of the 

 Attraction and apparent Repulsion of small Bodies 

 which move on the surface of Fluids. 4. On the Ad- 

 hesion of Discs to the Surface of Fluids. 5. On the 

 Figure of a large Drop of Mercury, and of the Depres- 

 sion of the Fluid in a Tube of Glass of a great Diame- 

 ter. The theoretical results obtained from the formu- 

 lae investigated by M. La Place agree, in a very won- 

 derful manner, with a series of experiments made, 

 at his request, by M. Gay Lussac, as will be seen from 

 the abstract of this theory with which the present Chap- 

 ter is concluded. 



Having thus given a brief account of the progress of 

 this branch of science, we shall now lay before our 



On th as. 

 cent of 



flullls m 

 & 



readers such additional information on the cohesion of OnC'apilUry 

 fluids and on capillary attraction, as may appear to be Attr * ctio| 

 necessary for completing the view of the subject which Cohesion of 

 might be expected in a work of this kind. Fluids. 



SECT. I. On Capillary Attraction. 

 1 . On the Ascent of Fluids in Glass Tubes. 



The experiments which were made during the last 

 century on the ascent of fluids in tubes of glass, differed 

 so widely from each other in their results, that no confi- 

 dence whatever could be placed in them. Some philoso- 

 phers did not scruple to assign different heights for the 

 same fluid and the same tube ; * and even if the pro- 

 per mode of cleaning the tube had enabled them to 

 observe the highest point to which the water rose, 

 they had no correct method of measuring the differ- 

 ence of level between the summit of the elevated co- 

 lumn and the surface of the fluid. The rise of the wa- 

 ter on the outside of the glass tube, rendered it parti- 

 cularly difficult to make such a measurement with the 

 accuracy which such delicate observations required. 



The first attempt to construct an accurate instrument, 

 appears to have been about the same time by Dr Brews- 

 ter and M. Gay Lussac, who, without any knowledge 

 of each other's invention, have employed the very same 

 principles for ascertaining, with the utmost accuracy, the 

 altitude of the fluid above its natural level. We do not 

 know which of these inventions are entitled to the merit 

 of priority. Dr Brewster's instrument was invented 

 some time in 1 806. An account of it was submitted 

 to the Royal Society of Edinburgh in February 1811, 

 and a drawing and description of it was published in 

 our article CAPILLARY ATTRACTION in 1812. Gay 

 Lussac's instrument must have been invented in J 807, 

 and probably much earlier, as the experiments con- 

 tained in La Place's second supplement, which appear- 

 ed in that year, seem to have been made with it ; but 

 so far as we know, no description of the instrument was 

 published till 1816, when it appeared in M. Biot's 

 Traite de Physique. 



This instrument is represented in Plate CCCXVI. 

 Fig. 3, where ABCD is a large cylindrical vessel of 

 glass for containing the liquid. It is placed upon a base, 

 which can be adjusted by the three adjusting screws 

 V, V, V, so that its orifice AB may be perfectly hori- 

 zontal, which can easily be ascertained by placing a 

 level upon it. The capillary tube TSH with which 

 the experiment is to be made, has a vertical motion in 

 a groove CC, perpendicular to a plate a b, which is 

 placed on the orifice AB of the glass vessel. By this 

 means the tube is kept in a vertical position, and it is 

 required only to measure the height of the column HS 

 above the level NN of the fluid. For this purpose Gay 

 Lussac employs a divided rule RR, which can be 

 placed in a vertical position by means of two adjusting 

 screws v v, and a plumb line FP. Along the divided 

 scale, a small telescope GH of a short focal length, and 

 with cross wires in the focus of the eye-glass, is made 

 to move by the screw nut *, so that the horizontal wire 

 may be made just to touch the lowest point of the sur- 

 face of the fluid. In order to determine the point H 

 which corresponds to the natural level of the fluid sur- 

 face, the plate a b, along with the tube TT, is pushed 

 aside till it is stopped by the side of the glass vessel, 

 (for if it were taken out, the surface NN would descend,) 

 and the bar / 1 resting upon the plate a' b' is placed on 



G 



fa c's instru- 

 mcnt for 

 measuring 



tubes. 

 p 



* Se our account of Carre's experiments, p. 46C- 



