166

Mifcellaneous Experiments on Heat.

difference between one body and another but what arifes from this circumftance ;-that these points being changeable by combination, give rife to a prodigious number of confequences different from thofe which obtain in the fimpler elements. We must refer to the original Effay for the more ample elucidation of these points, where chemical philofophers will fee the outline traced of an immenfe field of research, which, if it should not include the whole of the phenomena which have been defignated under the name of elective attraction, will certainly comprehend a large mafs of effects which never fail to prefent themselves in every natural change which comes under our notice.

CHAP. IV. The laft and concluding chapter contains a variety of mifcellaneous experiments. 1. If a thermometer with a long cylindrical bulb being at the temperature of the air in fummer, or any temperature above the freezing point, be plunged to half the length of its bulb in ice and water, the mercury will fal, in the tube only half as much as if the whole bulb had been immerfed. This experiment fucceeds equally well, when the fuperior half of the bulb is covered with a fheath lined with foft fur to prevent the communication of heat from the air during the experiment. It fhews that the upper half of the mercury is not cooled, or, in other words, that heat does not pafs downwards in fluids.-2. Ice-cold water standing on ice at the bottom of a thin tube of glass, may be boiled by holding the tube inclined over the flame of a candle applied near the upper end, and gradually removed lower down as the ebullition proceeds. In this way all the water in the tube may be brought into the most violent ebullition, to within a quarter of an inch of the ice, before this begins to be melted.-3. The radiant heat from a red hot iron bullet was not found to make its way downwards through liquid water, nor through melted tallow, nor melted wax. The experiment was tried on a thermometer about a quarter of an inch beneath the surface of each of thefe fluids. Thefe, facts are analogous to the obfervation of Scheele, who found that radiant heat does not pafs through glafs. When the red-hot ball was held over a cake of ice, the heat appeared to be tranfmitted by communication, though very flowly. The excavation in ice was deepest near the middle. The contrary was the cafe both in tallow and in wax. Count Rumford does not directly explain the cause of this remarkable difference. It is the only immediate fact I recollect having feen in his Effays, in proof of the affer. tion that water stands alone, with regard to the property of expanding in the lower degrees of refrigeration; and confequently of contracting as its temperature increases in that part of the scale. From this property it would follow, that a warm ftream of water muft defcend immediately beneath the buller, while an afcending current obtains near the circumference of the cavity. But if the expanfion of tallow be uniform from the earliest point of fufion, the heated fluid will not defcend, but will spread out fideways, and circulate in a contrary direction to the current in water; while the afcending ftream of cold fluid will be near the centre, and will defend part of the tallow in that region from being fused.4. Beautiful cryftals of fea falt were formed in brine ftanding on mercury in an open veffel for half a year. The Count makes it a queftion, whether the mercury contributed in any manner to this effect? It seems probable that it did not, and that evaporation equally flow and undisturbed would have afforded the fame refult in other circumftances.-5. A fimilar remark may be made on his obfervation of olive oil rendered colourless by expofure to the air for fix months upon brine, in a place where the fun's rays never entered. The fame effect hap-. pens to olive oil which remains in unclofed veffels in work-fhops.-6. Part of the laft-men

1

Circulation of a Fluid refembling Winds.

167

tioned colourless oil being imperfectly congealed, an attempt was made to caufe radiant heat to defcend through its mafs. It was expected that the tranfparent or melted portion of the oil would in this cafe have been hemifpherical, or at least of fome convex figure; but this did not happen, for the furface beneath the fluid or melted part continued flat. Whence the Count infers, that the fufion was effected merely by the heat abforbed or generated by the fides of the tumbler.

7. The concluding experiment of this Effay affords a ftriking refult of the circulation of fluids under different circumftances. When the inftrument defcribed in our Journal, I. 342, was placed by accident in the window of a room warmed by a German ftove, the difference of temperature, on the fide nearest the window from that oppofite the internal part of the room, was such as to keep up a conftant circulation of the fluid. This event induced the Count to fubftitute a box of plate glafs in the place of one of the panes of his window. This apparatus, which was 13 inches high, 10 inches wide, and one inch within from plate to plate, was half filled (and as I fuppofe afterwards completely filled) with the faline folution with pieces of amber floating in it, as defcribed at the place laft quoted. The Count expected to behold the currents as ufual in this new veffel; but to his great furprise they proved not vertical, but horizontal, exhibiting actual winds in oppofite directions, which fpringing up in the different regions of this artificial atmosphere, prevailed for a long time with the utmoft regularity, while the fmall particles of the amber collecting themfelves together formed clouds of the most fantastic forms, which being carried by the winds rendered the scene perfectly fafcinating.

Several fubordinate circumftances gave thefe motions a more intimate refemblance to the atmospherical phenomena. One remarkable appearance never failed to prefent itself regularly every day during the three weeks that the experiment was continued. The clouds,. after having been driven about all day by the different currents, in the liquid (of which there were fometimes as many as fix or seven running in oppofite directions at the fame time), never failed to collect themfelves together in the evening into large maffes, fometimes forming only one, and fometimes two or three ftrata at different heights, where they remained to all appearance perfectly motionlefs during the night.

The experiment was put an end to by the accidental breaking of the veffel.

VI.

An Account of certain Motions which small lighted IVicks acquire when fwimming in a Bafon of Oil; together with Observations upon the Phenomena tending to explain the Principles upon which fuch Motions depend. By PATRICK WILSON, F.R.S. Edin., and Professor of practical Aftronomy in the University of Glasgow *.

DEAR SIR,

INOV

Glasgow College, April 28, 1795,

NOW fit down to give you fome account of the little hydroftatical lamp which I fo briefly mentioned to you in a former letter. As I am far from being fure whether what I

* In a Letter to Mr. John Playfair, F. R. S. Edin. &c. inferted in the Edinburgh Transactions, vol. iv.

have

1.68

Account of a felf-moving Lamp.

- have to offer upon this fubject may be entitled to the notice of the Edinburgh Royal So ciety, so I will refer this point to your determination after you have had leisure to confider

the contents.

The phenomena treated of in the fequel were quite new to me a few months ago, and, fo far as I know, have not hitherto been attended to or defcribed by any body elfe. What I have called the hydroftatical lamp, consists of a small circular patch of common writing paper, about three-eighths of an inch in diameter, having about a quarter of an inch of soft cotton thread ftanding up through a puncture in the middle to ferve as a wick; and the phenomena in question are certain motions which fuch minikin lamps acquire when lighted and made to swim in very pure fallad oil.

A fhallow glass bafon, with fides rifing nearly perpendicular, or a common glafs falver, will conveniently contain the oil for these experiments. As foon as the lamp is lighted, it will immediately fail brifkly forward in some direction till it meets the fide of the vessel, and afterwards will take a circular courfe, always bearing up to the fides, and fo will perform many revolutions.

Sometimes the circulation is from right to left, and sometimes in the contrary direction, according as that point of the paper base, which in the direct failing kept always foremoft, turns away from the fide of the glafs, a little to the right or to the left hand of that which comes to be the point of contact. This turning away of what may be called the leading point of the base is distinctly obfervable by a partial rotation of the lamp round the wick as an axis, as foon as it arrives at the fide of the veffel. Sometimes, though rarely, the leading point itself attaches itself to the fide, and forms the vinculum, in confequence of the well-known corpufcular attraction between the elevation of oil around the base and that belonging to the fides of the glafs; and when the vinculum fo correfponds to the leading point, the lamp will be found to stand still without any tendency to circulate.

When the little wick has any fenfible eccentricity upon the circular paper base, the lamp will fail fo as to make that part of the bafe which lies nearest to the wick the ftern; and if the bafe of the lamp be clipped to an oval form, and the wick placed in the longer axis excentrical, that end of the base nearest the wick will also keep hindermoft, when the lamp fails across the falver. In the fame manner, if there be an equilateral triangle, having its wick in the perpendicular which bifects any of the fides, either the vertex or fide will become the ftern, and keep hindermost, according as the wick is placed nearest the one or the other. Lamps fo conftructed are found alfo to circulate upon their arrival at the fide of the veffel, when the leading point turns away from the glafs, as it commonly happens.

Whatever be the caufe of the failing of the lamp directly forward, the perpetual circulation after it arrives at the fide feems to proceed from the force which formerly impelled it ftill acting in the fame manner, but in a direction inclined to that of the corpufcular attraction which forms the vinculum; and it is evident that this inclination will be greater or lefs according as the leading point is more or lefs averted from the glass. When it,fo happens that the leading point and vinculum coincide, it should feem that both forces juft now mentioned muft urge the lamp in a direction perpendicular to the fide of the glass; in which cafe it must stand ftill, agreeable to observation.

The next thing which I had occafion to take notice of when the lamp failed in a direct course, was a feemingly very active repulfion between its ftern and the oil at the surface con- |

tiguous

Principle of Motion in a floating Lamp.

169

tiguous to it. This became manifeft when very fine charcoal duft was lightly scattered around the lamp. As it then proceeded in its courfe, it marked out a fpreading or diverging wake behind it entirely clear of all duft, in confequence of the particles being chafed backwards and laterally with a motion much more than merely relative.

Defirous of learning how this difpofition of the duft would take place when the lamp was ftationary, I conftructed one of a fine wafer, and with an excentric wick confifting of a foft cotton thread doubled; and to prevent the wafer or bafe from catching fire I coated its upper furface with gold leaf. When this was made to reft immoveably upon the oil, the duft retired in all directions fo as to leave the space adjacent to the wafer quite free from every particle. But here it was obfervable, that this difperfion of the dust, by the feeming repulfion of the bafe of the lamp, was much more rapid at that fide which lay nearest to the wick than at any other part, and leaft of all fenfible at the fide diametrically oppofite.

The circumftances laft mentioned feem fufficiently to account both for the progressive motion of the lamp, and for the general law of this motion formerly defcribed. For, regarding this difperfion of the duft as yet only in a general way, and as the effect of fome repulfion between the bafe and the oil contiguous to it, the facts above mentioned plainly indicate, that in all cafes this repulfion is strongest at that part of the base nearest the wick or flame; and as action and reaction are equal and contrary, the lamp must therefore be impelled, in the direction of a line drawn through the wick, towards that part of the bafe most remote from it, and where the reaction is the leaft.

But in order to obtain a ftill more competent knowledge of the phyfical caufe of these motions, it feemed now neceffary to enquire more particularly into this apparent repulfion, between the base of the lamp and the furrounding oil, as indicated by the difperfion of the duft in the manner above defcribed: and here the following confiderations prefented themfelves:

The oil in the bafon, when of an uniform temperature, has all its parts in a state of equilibrium and of reft. When the lamp is lighted, it is evident we have a very active cause introduced tending to deftroy that equilibrium. This caufe is the flame which broods over a small portion of the oil, and is feparated from it only by the intervention of a piece of paper or a wafer. The oil in fuch circumftances, in confequence of being violently heated, must suddenly increase in volume, and must now, on account of the decrease of its specific gravity, be preffed upwards by a force fufficient to raise part of it above the general level. But this heated portion of oil, in its endeavour to rife up, will meet with a resistance equal to the weight of the incumbent lamp, which will determine it, in feeking a vent, to flide out from under the base in a thin fuperficial ftream; and it seems to follow with equal certainty, that this constant stream will flow most rapidly and moft copiously towards that side of the bafe of the lamp where the refiftance is leaft, or where it has the shortest way to press forward; that is, from under the wick or flame to the edge of the bafe which is the nearest, according to what we have seen to be agreeable to the phenomena. But, from the laws of motion, it is certain that the re-action of this stream of rarefied oil, thus iffuing most rapidly and moft copiously from a particular fide of the bafe, muft impel the lamp in the contrary direction, and make it fail in the manner we have seen. It may further be remarked, that the heated oil fo retreating from the flame, and endeavouring to rife fomewhat above the general level, in confequence of its diminished fpecific gravity may more or lefs lift up that fide VOL. II.-JULY 1798.

Ꮓ

of

170

Account of a felf-moving Lamp.

of the base nearest the wick, and aid the re-action of the recoiling ftream, by making the lamp fail in the oppofite direction, as it were down hill.

That the rarefied oil under the bafe has really a conftant tendency to rife above the general level, feems undeniable from the following facts; namely, that after any of the lamps has burned a little while, and has got its base soaked with the oil, as foon as the flame is blown out the lamp finks to the bottom; and even a lamp with its bafe made of a thin lamina of talc fails very well till the flame is extinguifhed, and then it immediately finks.

Agreeable to the explanation which has now been attempted, I found that when a to pical heat was applied to the surface of the oil, by bringing the point of a poker dully red hot nearly into contact, there was foon produced a fuperficial stream or efflux from the iron in all directions, which cleared the face of the oil from the charcoal duft in a wider and a wider circle, till at last the whole particles were crowded together at the confines of the bafon.

When the oil in this experiment was fhallow, having gold leaf beat into very minute parts mixed with it, an oppofite ftream was obferved below fetting in towards the poker in all directions, and then rising upwards. But this general tendency of all the parts of the fluid, of moving in queft of an equilibrium, is illuftrated in a very entertaining manner as follows: Into a tea cup or punch glass nearly filled with pure water, pour a dessert spoonful of very clear fallad oil with minute particles of gold leaf in it. If the water be cold, the oil when poured on at the centre, leifurely and continuedly, will reft upon the surface in the form of a lens, and remain infulated and equidiftant from the fides of the veffel. A little lamp, when put upon this lens of oil and lighted, will fail and circulate as longer ones do in a bafon. If it be now made to stand still, it is very amufing to obferve the minute particles of the gold perpetually thrown out brifkly at the ftern in the fuperficial current, whilft the particles in the fund of the lens creep in all directions towards the lamp, and at last rise up under the bafe towards the flame, as the great centre of attraction, till they are caught by the retreating fuperficial ftream, in which they rapidly trend off to fome distance, when again they fink to renew the circulation.

When a patch of paper, or a wafer, or fuch light body, fwims upon the oil in the bafon, the point of a hot iron held near to it makes it flit its place, and move away by a feeming repulfion; but in reality by the heat generating a fuperficial ftream flowing from the iron in all directions.

Again, if upon oil of turpentine, æther, alcohol, or any of the inflammable fluids poffeffing much tenuity, you throw a wafer much heated, it will immediately glide away and continue in motion till it cools; when the stream which iffued from fome part of it most copioufly ceafes. Double rum, melted tallow, bees-wax, and rofin, alfo afford the fame continued efflux at the furface upon a topical application of heat, and the fame phenomena as the oil docs when little lamps are made to fwim in them. It is fomewhat remarkable, however, that though the inflammable fluids all agree in this, yet the topical application of heat at the furface of water does not produce fimilar effects.

For, if the point of a poker nearly red hot be held very close to the furface of water in a bafon, the particles of the charcoal duft do not at all glide away, as they do in the cafe of oil, but feem to acquire only a flow irregular circular motion, which in time fpreads wider,