4G8 



HYDRODYNAMICS. 



On Capillary 



Attraction 



and the 



Cohesion of 



Bullfinger's 

 experi- 

 ments. 

 A. D. 1T2T. 



Theory of 

 Daniel 

 Bernoulli, 

 172T. 



a capillary tube, when its orifice is immersed in the 

 fluid, has its gravity taken off by the attraction of the 

 periphery with which its upper surface is in contact : 

 Hence it must necessarily rise higher, partly by the 

 pressure of the stagnant water, and partly by the at- 

 traction of the periphery immediately above that which 

 is already contiguous to it. These opinions Dr Jurin 

 endeavoured to support by the experiment represented 

 at E, F, G in Fig. 5. of Plate CX. and described un- 

 der CAPILLARY ATTRACTION, p. 407. (See Phil. 

 Trans, vol. xxx. p. 739.) In a subsequent paper he in- 

 quires into the cause of the suspension of water in 

 tubes of glass, and seems to adopt the opinion, that the 

 cohesion may depend on the pressure of a medium 

 subtle enough to penetrate the receiver. " For though 

 such a medium," says he, " will pervade the pores of 

 the water as well as those of the glass, yet it will act 

 with its entire pressure on all the solid particles, if I 

 may so call them, of the surface of the water in the 

 cistern; and whereas so many of the solid particles of 

 the water in the tube as happen to lie directly under 

 the solid particles of the water above them, will there- 

 by be secured from this pressure, and consequently 

 there will be a less pressure of this medium on any 

 surface of the water in the tube below the capillary, 

 than in an equal surface of the water in the cistern ; 

 so that the column of water suspended in the tube 

 may be sustained by the difference between these two 

 pressures." 



The subject of capillary attraction was treated at 

 great length by George Bernhard Bullfinger, in a disser- 

 tation entitled, De Tubulis Capillarium, dissertatio cx- 

 perimentalis, which appeared in the Commetilarii Acad. 

 PelropolitancE for 1 727. This paper contains an examina- 

 tion of the different hypotheses which had been em- 

 ployed to explain the phenomenon of capillary .ttrac- 

 tion, and several new experiments illustrative of his 

 own opin?on. He found that the relative ascent of spirit 

 of wine, red wine, and water, were as 4, 7 and 12. He 

 seems, upon the whole, to prefer the hypothesis of Dr 

 Jurin, although he states a number of difficulties which 

 attend it. Dr Jurin replied to this paper of Bullfinger's 

 in the volume of the Commentarii Acad. Peiropolitatue 

 for 1728 ; and his paper is published with the annota- 

 tions of Bullfinger. After examining all the objections 

 which had been stated, and apparently to the satisfaction 

 of Bullfinger, Dr Jurin begs that he will no longer 

 consider his explanation as a hypothesis, but as a true 

 and established theory.* 



When M. Bullfinger exhibited his experiments before 

 the Academy of St Petersburgh, his colleague, the ce- 

 lebrated Daniel Bernoulli, who was at that time un- 

 acquainted with the speculations of his uncle on the 

 same subject, proposed a new theory of capillary at- 

 traction. In order to get over the difficulty respecting 

 the ascent of the water under the receiver of an air 

 pump, he ascribed the phenomenon, not to the un- 

 equal pressure of the air as his uncle had done, but to 

 the unequal pressure of an ethereal fluid. He consider- 

 ed the base of the fluid as contiguous to the surface 

 of the water ; but he supposed that it was not so full 

 at the sides of the pipe as in its axis ; or, in other 

 words, that the aether stood at a greater distance from 

 the glass than water did, or was less dense in the neigh- 

 bourhood of the glass. This effect he attributed to the 



1737 



particles of the aether being greater than tho?; 1 of water. On Capillary 

 This hypothesis, which differs in no respect from that Attraction 



of James Bernoulli, excepting in the substitution of r f nd . the . 

 ., . , . f *> . . Cohesion ot 



ictner tor air, has the advantage of surmounting the Fluids. 



difficulty already mentioned. But, in other respects, .^-yw/ 



it is more inadmissible. Daniel Bernoulli attempted 



to shew that it explained many of the common experi- 



mt'nts, and that the proportion between the magni- 



tude of the particles of different fluids might be dedu- 



ced from the height of their ascent in capillary tubes. 



He inferred, from a rude and erroneous experiment, 



that the particles of mercury were twice as small as 



those of water. 



An excellent dissertation, entitled Dissertatio P/iysi- E xper j. 

 ca de Tttbis Capillaribus Vilrcis, was published by M. me nts of 

 Muschenbroek, which contains a great variety of inter- Muscheo. 

 esting experiments upon this subject. He has committed broek. 

 a mistake, however, in maintaining that the height of Born IC99 ' 

 ascent increases with the length of the tube. The D 

 constant quantity for water, as deduced by Dr Young, 

 from his best experiments, is 0.392. 



In the year 1736, Josiah Weitbrecht published a va- Weit- 

 luable paper in the Commentarii Acad. Pefropolitance, en- brucht's 

 tilled Tentamen Theoricc qua ascensus aqux in lulls ca- 

 pillaribus explicalur. He shews that water is more 

 strongly attracted by glass than it is by its own parti- 

 cles ; that the sphere of activity of the attraction of the 

 glass is extremely small, that is, the action of the glass 

 does not extend to the axis of the tube ; that the water 

 must be partly supported by the mutual attraction of its 

 own particles ; that the water in the capillary tube is 

 drawn downwards, not only by its own gravity, but by 

 the attraction of the water in the vessel ; that the water 

 in the tube is elevated by the attractive force of the 

 whole internal surface of the tube successively ap- 

 plied ; but that it is suspended solely by the action of 

 the ring of glass immediately above the fluid column. 

 M. Weitbrecht considers the force which suspends the 

 water as represented by Q Q', Q representing the force 

 with which the water is attracted to the glass, and Q' 

 the force with which it is attracted downwards by the 

 water in the vessel ; and as Q is greater than Q' in water 

 and most fluids, the quantity Q Q' is affirmative, and 

 the water rises above its level. When the tube is taken 

 out of the water, the force Q' he considers as vanishing, 

 and therefore the remaining force is allowed to act with- 

 out opposition, and consequently the water rises to a 

 greater height in the tube. In mercury Q' is greater 

 than Q, and therefore the expression is negative, and 

 the fluid consequently sinks below its level. M. Weit- 

 brecht made the following experiments on the ascent 

 of water. 



Diameter of the tube. Height of ascent. 

 English Inches. Inches. 



0.06 0.72 . 



0.045 0.95 . . 



0.045 



0.92 



Constant quantity. 



. 0.0432 

 . 0.04275 



0.04140 



* " Si iita," lays he, " quod speramus, Cl. Bullfinger satisfecerint, pollicemur nobis, pro candore et sequitate Viri humanissimi 

 eum in posterum explicationem hanc phenomenon non amplius pro kypotkeei, sive/uuVi ingenii commtnto, aed fro vcru et bidubita to- 

 rundem thtoria habitutura." Comment. Fetrop. 1728, p. 201, 292. 



