480 



HYDRODYNAMICS. 



in a 

 bason of oil- 



OnCpiHary fore wiping the ivory, the attraction commenced soon- 



attraction er, and sometimes exhibited itself at the very first, with- 



Co'helion out being preceded by any sensible repulsion. This 



of I'luitls. experiment was repeated several times with the same 



s-^-y-^ result. 



Another series of phenomena, which indicate apparent 

 attraction and repulsion, are seen in the motion of small 

 lighted wicks when swimming in a bason of oil, and 

 in the motion of camphor upon the surface of water. 

 Although these phenomena are not produced by ca- 

 pillary attraction, yet we shall give a short account 

 of them at present, on account of their general simila- 

 rity. 



Dr Wilson's The phenomena of lighted wicks were carefully ob- 

 experiments served, and minutely described, by Professor Wilson 

 on the mo- of Glasgow in the Transactions of the Royal Society of 

 Jj Edinburgh. His Hydrostatical Lamp, as he calls it, 

 wfcksswira- cons ' s ts f a small circular disc of common writing pa- 

 per, about % of an inch in diameter, having about a 

 quarter of an inch of soft cotton thread rising up. 

 through a puncture in the middle of the disc to answer 

 the purpose of a wick. If this wick is lighted, and the 

 disc placed in a shallow glass vessel filled with pure 

 salad oil, it will immediately sail briskly forward in 

 one direction till it meets the side of the vessel, and 

 will afterwards take a circular course, always bearing 

 up to the sides, and will thus perform many revolu- 

 tions. The circulation is sometimes from right to left, 

 and sometimes from left to right. When the wick is pla- 

 ced out of the centre of the disc, it will sail to that part of 

 the disc which is farthest from the wick, and if the disc 

 is made of an oval form, and the wick placed in one of its 

 foci, the disc will sail in the direction of the nearest ex- 

 tremity of the transverse axis. Dr Wilson observed avery 

 active repulsion between the stem of the disc and the 

 oil of the surface contiguous to it. When fine charcoal 

 dust was scattered around the disc, it left behind it a 

 diverging wake clear of all dust. Other fluids, such 

 as oil of turpentine, ether, alcohol, or any of the in- 

 flammable fluids possessing much tenuity, also double 

 rum, melted tallow, bees wax, and rosin, exhibit the 

 same effects when the discs float upon their surface. 



Dr Wilson accounts for these phenomena in the fol- 

 lowing manner: When the oil has an uniform tempera- 

 ture, all its parts are in equilibrio; but when the lamp 

 is lighted, the oil below the disc being most heated, 

 will expand itself, and will be raised above the general 

 level, from the diminution of its specific gravity, and 

 the unbalanced upward pressure of the fluid. The 

 weight of the disc will therefore press down the oil, 

 or even the weight of the oil itself will cause it to rise 

 as it were from below the wick in a thin superfi- 

 cial stream. Hence Dr Wilson conceives, that this 

 constant stream will flow most readily and copiously 

 towards that side of the base of the lamp where the re- 

 sistance is least, or where it has the shortest way to press 

 forwards, that is from under the wick or flame, or the 

 edge of the disc, which is the nearest. The reaction of 

 the stream of rarefied oil rising most rapidly and most 

 copiously from one side of the disc, will therefore im- 

 pel the lamp in the contrary direction. When the 

 discs are soaked with oil, or when they are made of a 

 thin plate of talc, they always sink to the bottom as 

 soon as the flame is -extinguished. If a wafer much 

 heated is thrown upon any of the fluids above men- 



Ventuti's 

 experinienU 

 on the disso- 

 lution and 

 motion of 

 camphor 

 floating o* 

 water. 



tioncd, it will immediately glide away, and continue in On Capillary 

 motion till it cools.* .uu.vmm 



The singular motions of pieces of camphor floating ,, a ," <l . the . 

 i P , & Cohesion of 



upon the surface of water, have been long observed ; Fluids 



but they were never completely described and explain- "^r' 

 ;1 till M. Venturi published in the Memoirs presented to 

 the Institute of -France his ingenious memoir, entitled 

 Precis de qtirli/nes experiences sur la section one des 

 cylindres de ciim/Jire eprouvenl a la surface de t'cau el 

 reflexions sur tes mouvemens qni accompagmmt cetle sec- 

 tion. 



Having cut some pieces of camphor into the shape 

 of small cylinders, a line in diameter, and an inch high, 

 he fixed each of them to a base of lead, and placed 

 them vertically in plates. He then poured water into 

 the plates, till it reached about half way up each cy- 

 linder. After two or three hours the cylinders began 

 to diminish at the place where they were cut by the 

 surface of the water, and after about twenty- four hours 

 they were entirely cut through into two parts, none of 

 which had suffered any sensible diminution. 



He next took three pieces of camphor, each of which 

 weighed twenty-four grains, and he placed one of them 

 in dry air, another in water, and the third on the 

 surface of water. After four days the piece on the sur- 

 face of the water was entirely dissipated, while each of 

 the other two had lost only four or five grains. 



M. Venturi next placed some of his camphor cylin- 

 ders on the surface of water, in vessels of different 

 sizes, and he always found that the cylinder was cut 

 through soonest in those vessels that presented the 

 greatest surface. This singular fact arises from the 

 camphor being dissolved by the water, and extending 

 itself over all the fluid surface, when it is the more 

 readily evaporated by its coming into contact with a 

 greater quantity of air. The dissolution of the cam- 

 phor may be seen detaching itself from the cylinder 

 in the form of a very transparent liquid oil, and spread- 

 ing itself over the whole surface of the water. When 

 in the contour of the cylinder, there is some place which 

 furnishes this oil more abundantly ; if small light sub- 

 stances are thrown upon the surface, they are repelled 

 from this place with great briskness, and then turning 

 round, they come back to the same place, and again 

 enter the current, where they continue to circulate in 

 this manner. If a small piece of camphor previously 

 wetted at its extremity approaches the margin of the 

 vessel, and then touches the vessel itself, it deposits a 

 fluid visible to the eye. This fluid is oily; and on at- 

 taching itself to the vessel, it destroys its capillary at- 

 traction for the water, and the water retires from it and 

 becomes convex at this place. When the piece of cam- 

 phor is taken away, the water does not return to its 

 place till the oily liquor is evaporated. When the cy- 

 linders of camphor are half immersed in water, the 

 oily liquor which issues from it destroys the cohesion, 

 between the water and the cylinder, and a small de- 

 pression takes place round the cylinder. The dissolu- 

 tion stops for a moment till the oily liquor expanded 

 over the water has evaporated. The water then re- 

 turns to its elevated state round the cylinder ; the cam- 

 phor is dissolved and diffused ; and the same opera- 

 tions are repeated. 



The motions of small pieces of camphor observed 

 by M. Romieu are produced by the mechanical reac- 

 tion of the jet of dissolution against the camphor ; and 



See the Edinbttrgh Transaction!, voL iv. p. 144, &c. 



