HYDRODYNAMICS. 



Hooke's 

 fxperi- 

 qjcnts. 

 166T. 



Investiga- 

 tions of 

 Vossius. 

 1606. 



the Vhti Observation of his Micrographia, which 

 appeared in IttJT. This observation "is entitled, On 

 Small Glass Canes, and contains his most mature opi- 

 nions on the subject. He states that the water, when 

 it enters small capillary tubes, rises rapidly to the 

 height of 6 or 7 inches ; that when the tube is extreme- 

 ly fine, it ascends slowly to a much greater height; 

 and that he had never patience to wait till it rose high- 

 er than 21 inches, which must have been in a pipe, 

 whose internal diameter was about the T^^th part of 

 an inch. He defines the term Congruity, which may be 

 considered the same as affinity, as that " property of a 

 fluid body, whereby any part of it is readily united 

 with any other part, either of itself, or of any other simi- 

 lar fluid or solid body ; and Incongruity, to be that pro- 

 perty of a fluid, by which it is hindered from uniting 

 with any dissimilar fluid or solid body.'' Dr Robison, 

 and some other authors, are therefore mistaken in claim- 

 ing for Dr Hooke the merit of explaining the pheno- 

 mena of capillary attraction by affinity, by which they 

 meant the affinity of water to glass. Dr Hooke indeed, 

 employs a term the same as this in his explanation of 

 these phenomena ; but it is employed for a quite dif- 

 ferent purpose ; for he supposes that the water rises in 

 the tube, not because it is attracted by the glass, but be- 

 cause there is a greater affinity between water and glass 

 than between air and glass, in consequence of which, the 

 column of air within the tube is not capable of balan- 

 cing the corresponding atmospherical column without. 

 " For since the pressure," says he, " of the air every way 

 is found to be equal, that is, as much as is able to press 

 up and sustain a cylinder of quicksilver of 2^ feet high 

 or thereabouts ; and since of the pressure so many more 

 degrees are required, to force the air into a smaller than 

 into a greater hole that is full of a more congruous 

 fluid ; and, lastly, since these degrees that are requisite 

 to press it in, are thereby taken off from the air within, 

 and the air within left with so many degrees of pres- 

 sure less than the air without; it will follow, that the air 

 in the less tube or pipe will have less pressure against the 

 superficies of the water therein, than the air in the big- 

 ger. The conclusion, therefore, will necessarily follow, 

 viz. that this unequal pressure of the air, caused by its in- 

 gress into unequal holes, is a cnuse sufficient to produce 

 the effect, without the effect of any other concurrent ; 

 and therefore is probably the principal (if not the on- 

 ly) cause of these phenomena. This, therefore, being 

 thus explained, there will be divers phenomena expli- 

 cable thereby : as, the rising of liquors in a filtre; the 

 rising of spirit of wine, oil, melted tallow, &c. in the 

 wick of a lamp, though made of small wire, threads of 

 asbestos, strings of glass, or the like ; and the rising of 

 liquors in a sponge, pieces of bread, sand, &c. ; perhaps 

 also the ascending of the sap in trees and plants, through 

 their small and some of them imperceptiblepores,at least 

 the passing of it out of the earth into their roots.''* This 

 hypothesis ofDr Hooke's, which was received at the 

 time with great applause, was afterwards shewn to be 

 unsatisfactory and inconsistent with experiments by 

 Roger Cotes.t 



In the year 1666, the learned Isaac Vossius publish- 

 ed at the Hague his work entitled, De Nili et Aliorum 

 Flumiiium Origine, in the second chapter of which he 

 describes the phenomena of capillary attraction, and 

 endeavours to account for them by a theory which ap- 

 proaches more nearly than any other which hadbeengiven 

 to the true theory of the action of capillary tubes. Since 



Fabri. 

 Born 160T. 



water, says he, is by its very nature viscid, it adheres to On Capillary 

 every thing which it touches, so that it adheres to glass, Attraction 

 and is sustained by the glass. But since the water is sus- ., " nd tlle 

 tained by the action of the glass, it does not press upon 

 the water below it, as the same weight cannot press in 

 twoplaces, and as nobody can be heavier than itself. The 

 portion of water therefore which enters the tube, loads 

 the glass tube, to the sides of which it adheres, and is 

 destitute of weight in respect of the subjacent water. 

 Hence it follows, that if capillary tubes are immersed 

 in water, and then taken out of it, the water which 

 has entered them will not all flow out of the tube, but 

 as much will remain as the surface of the tube can sus. 

 tain. From this hypothesis Vossius concludes, that 

 water will rise higher in narrow than in wide tube. , 

 because the narrow tubes, in proportion to their capaci- 

 ty, present more points of contact of adherence to the 

 water, and that mercury being destitute of viscidity, 

 will not adhere to glass, and will therefore sink below 

 its natural level in capillary tubes. 



The first person in France who repeated these experi- F.xperi. 

 ments, and attempted to investigate their cause, was m ts f 

 M. Honore Fabri, a learned Jesuit, who was bora 

 at Bellay near Lyons, in the year 1607. In the 

 year 1669, he published a work entitled, Dialcui 

 Physici, the fourth chapter of which is entitled, DK 

 humoris elevatione per Canaliculum. In this chapter, 

 he observes that water, whether hot or cold, ascends 

 above the level of the water in the vessel; that it ri- 

 ses to a greater height in narrow than in wide capillary 

 tubes ; and that the water ascends highest in tubes of 

 the same diameter when the tubes extend farthest 

 above the surface of the water; that the water raised 

 by capillary attraction will never flow out of the top 

 of the tubes, however short ; that the water will rise 

 higher in a wet tube than in a dry one ; that the wa- 

 ter will not rise in a tube if the finger is placed upon 

 the upper end of its bore previous to immersion ; 

 and that in two concentric tubes, the water will rise 

 sometimes higher and sometimes lower in the widest 

 of the two tubes, according as the difference of their 

 diameters is less or greater than the diameter of the in- 

 ner tube. In explaining these phenomena, he main- 

 tains, that the external air, acting as a compressed 

 body, has free access to press upon the surface of the 

 water exterior to the tube, whereas it does not act so 

 freely upon the surface of the water in the tube, and 

 therefore the fluid will rise with a force proportional to 

 the difference of these pressures. The cause of this 

 unequal pressure of the air Fabri supposes to be, that 

 only an inverted cone of air touching the fluid in the 

 tube with its vertex, and having the upper orifice of the 

 tube for its base, can press upon the surface contiguous 

 to its vertex. 



The celebrated Alphonso Bovclli has attempted to Experi- 

 explain the phenomena of capillary attraction in his ments of 

 Treatise De Motionibus a Gravitate nalurali pcndente, ' 

 which was published at Lyons in 1670. He seems, 

 like Vossius, to ascribe the elevation of the water to its 

 adhesion to the sides of the tube, and he considers the 

 attractive force of the tube as extending to the particles 

 of water placed in its axis. " In cavitatibus," says he, 

 " subtilium fistularum interims aqua- contactus grand- 

 is est et amplus, respectu illius aqnse molecula; ibidem 

 existentis : ergo subito ac infimum fistulas orificium 

 attingit aquam, efficitur, in ejus interna et cava perime- 

 tro efficacissimus contactus a cujus adhesione fulciri 



* Hooke's Aficrographia, p. 21. 



t Se Cotes' Hydrostatieal Lectur<s, Lect. XI. Lond. 173A 



