Efted of a very thin plate of copper, a little more than an inch fquare, which was faftened to one end of a slender harpfichord wire about ten inches long. To the middle of this was fixed an agate cap, fuch as is commonly used for small mariner's compaffes, after the manner of which it was intended to turn; and at the other end of the wire was a middling fized fhot corn, as a counterpoife to the copperplate. The inftrument had also fixed to it in the middle, at right angles to the length of the wire, and in an horizontal direction, a fmall bit of a very flender sewing needle, about one third or perhaps half an inch long, which was made magnetical. In this ftate the whole inftrument might weigh about to grains. It was placed on a very fharp-pointed needle, on which the agate cap turned extremely freely; and to prevent its being difturbed by any motion of the air, it was inclofed in a box, the lid and front of which were of glafs. This box was about 12 inches long, 6 or 7 inches deep, and about as much in width; the needle standing upright in the middle. At the time of making the experiment, the box was fo placed, that a line drawn from the fun paffed at right angles to the length of it; and the inftrument was brought to range in the fame direction with the box, by means of the magnetical bit of needle above mentioned, and a magnet properly placed on the outfide, which would retain it, though with extremely little force, in any fituation. The rays of the fun were now thrown upon the copperplate from a concave mirror of about two feet diameter, which, paffing through the front glass of the box, were collected into the focus of the mirror upon the plate. In confequence of this the plate began to move, with a flow motion of about an inch in a fecond of time, till it had moved through a space of about two inches and a half, when it ftruck against the back of the box. The mirror. being removed, the inftrument returned to its former fituation by means of the little needle and magnet; and the rays of the fun being then again thrown upon it, it again began to move, and ftruck againft the back of the box as before; and this was repeated 3 or 4 times with the fame fuccefs. The inftrument was then placed the contrary way in the box to that in which it had been placed before, fo that the end to which the copperplate was affixed, and which had lain, in the former experiment, towards the right hand, now lay towards the left; and the rays of the fun being again thrown upon it, it be gan to move with a flow motion, and ftruck against the back of the box as before; and this was repeated once or twice with the fame fuccefs. But by this time the copperplate was fo much altered in its form, by the extreme heat which it underwent in each experiment, and which brought it nearly into a ftate of fufion, that it became very much bent, and the more fo as it had been unwarily fupported by the middle, half of it lying above and half below the wire to which it was faftened. By these means it now varied fo much from the vertical pofition, that it began to act in the fame manner as the fail of a windmill, being impelled by the ftream of heated air which moved upwards, with a force fufficient to drive it in oppofition to the impulfe of the rays of light. If VOL. XIII. PART I.

we impute (fays Dr Priestley) the motion produ ced in the above experiment to the impulfe of the rays of light, and fuppofe that the inftrument weighed ten grains, and acquired a velocity of one inch in a fecond, we shall find that the quantity of matter contained in the rays falling upon the inftrument in that time amounted to no more than one 1200 millionth part of a grain, the velocity of light exceeding the velocity of one inch in a fecond in the proportion of about 1,200,000,000 to 1. The light was collected from a furface of about three fquare feet, which reflecting only about half what falls upon it, the quantity of matter contained in the rays of the fun incident upon a square foot and an half of furface in one fecond of time, ought to be no more than the twelve-hundred millionth part of a grain, or, upon one square foot only, the 1800 millionth part of a grain. But the denfity of the rays of light at the furface of the fun is greater than at the earth in the proportion of 45,000 to 1; there ought, therefore, to iffue from one fquare foot of the fun's furface in one fecond of time, in order to fupply the wafte by light, one 40,000th part of a grain of matter; that is, a little more than two grains in a day, or about 4,752,000 grains, or 670 pounds avoirdupois nearly, in 6000 years; a quantity which would have fhortened the fun's femidiameter no more than about ten feet, if it was formed of the denfity of water only." The Newtonians, befides the answer juft given to the moft formidable objections of their opponents, have endeavoured to prove the impoffibility of light being a vibration in any fluid." Sir Isaac, in his Principia, demonstrates, that no rectilinear motion can be propagated among the particles of a fluid unless thefe particles lie in right lines; and that all motion propagated through a fluid diverges from a rectilinear progrefs into the unmoved fpaces. Hence he concludes, " a preffure on a fluid medium (i. e. a motion propagated by fuch a medium beyond any obftacle, which impedes any part of its motion) cannot be propagated in right lines, but will be always inflecting and diffufing itfelf every way, to the quiefcent medium beyond that obftacle. The power of gravity tends downwards; but the preffure of water rifing from it tends every way with an equable force, and is propagated with equal eafe, and equal strength, in curves, as in ftraight lines. Waves, on the furface of the water, gliding by the extremes of any very large obftacle, inflect and dilate them felves, ftill diffufing gradually, into the quiescent. water, beyond that obftacle. The waves, pulfes, or vibrations of the air, wherein found confifts, are manifeftly inflected, though not fo confiderably as the waves of water; and founds are propagated with equal eafe, through crooked tubes and through ftraight lines; but light was never known to move in any curve, nor to inflect itself ad umbram." To this Mr Rowning adds another proof: "The Cartefian notion of light (fays he) was not, that it is propagated from luminous bodies by the emission of smail particles, but that it was communicated to the organ of fight by their preffure upon the materia fubtilis, with which they fuppofed the univerfe to be full. But according to this hypothefis, it could never be

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courfe in many different ways when acting in the common atmosphere, but we have no reafon to fuppofe that it would be the fame in a perfect vacuum; at leaft we have no right to lay it down as a principle to argue from, unless it were verified by experience. His 3d pofition, that the light emitted by combustible bodies formed part of their fubftance before combuftion, feems ftill worfe founded; for inftead of being fixed in folid fubftances, all the light and heat proceeding from combuftion feem entirely to come from the air. See COMBUSTION, FIRE, FLAME, &C.

Thefe fubftances. Hence he concludes, that when the attractive force by which the feveral rays of light are attached to a body is weakened, fome of thofe rays will escape fooner than others; it being evident that those which are detained by the smallest power will fooneft go off when the general attractive force is weakened. This he illuftrates by the example of a mixture of spirit of wine, water, and other more fixed fubftances. The application of a gentle heat will carry off the fpirit of wine only; a heat not much greater will evaporate the fpirits and water mixed together; and a ftill greater degree will carry off a mixture of all the particles together. "In like manner (fays he), when the furface of a combustible is in a ftate of decompofition, those parts of it which are the leaft fixed, or which are united with the leaft force, will be separated firft. Among these the indigo rays of light will make the earliest appearance. By increafing the heat, we fhall mix the violet with the indigo: by increafing it ftill more, we shall add the blue and the green to the mixture, till at length we reach that intenfity of heat which will caufe all the rays to escape at the fame instant, and make the flame of a combuftible perfectly white. By examining the flame of a common candle, we may obferve, that its lowest extremities, or the part in which the black colour of the wick terminates, difcharges the leaft heat; and that, as the vertex of the flame is approached, a fucceffive order of parts is paffed through, in which the loweft is continually adding to the heat of that which is just above it, till we come to the top of the flame, near which all the heat is collected into a focus. At the loweft extremity however, where the heat is inconfiderable, a blue cofour may always be obferved; and from this ap pearance, amongst others, I think it may be concluded, that the blue rays are fome of those which efcape from combuftibles in an early period of their decompofition; and that if the decompofition could be examined in a period ftill more early, the colour of the flame would be violet." From thefe and other facts Mr Morgan infers, "That light, as a heterogenous body, is gradually decompofed during combuftion: that the indigo rays efcape with the leaft heat, and the red with the greateft; and from this again he explains the reason why flames affume different colours." He concludes the fubject with a criticism upon Sir Ifaac Newton's definition of flame, viz. that it is a vapour heated red hot. In his opinion, "flame is an inftance of combuftion whofe colour will be determined by the degree of decompofition which takes place. When very imperfect, only the moft refrangible rays will appear. If very perfect, all the rays will appear, and its flame will be brilliant in proportion." Thus we have a most elaborate theory for folving phenomena which feem not easily to admit of any folution. The data uppon which he builds his fyftem are altogether hy. pothetical. That light is fubject to the laws of attraction, cannot be proved, unless we could examine it independent of any other fubftance whatever; that is to fay in a perfect vacuum. But in the most perfect vacuum that can be formed, we are far from being certain that no other matter is prefent. Light is inflected and turned out of its

(8.) LIGHT, MR MORGAN'S EXPERIMENTS ON ELECTRICAL. In the fame paper Mr Morgan has fome curious obfervations upon the electric light. There is neither fluid nor folid, he says, through which the electric fluid in its paffage will not appear luminous, if we do not make the quantity, through which it has to pafs, too great. In his experiments on fluids, he puts them into a tube about 3-4ths of an inch diameter and 4 inches long. The orifices are then flopped up with two corks, through which two pointed wires are thruft, so that the points may approach within of an inch of each other; and in this cafe the electric matter which paffes through the fluid is always luminous, provided a fufficient force be ufed. The experiment, however, is dangerous, unlefs great care be taken; and the tube, unless it be very ftrong, will be broken by a very flight difcharge. With acids the experiment fucceeds more difficultly; they must be put into capillary tubes, and the wires placed very near to each other. Some of his experiments with gold leaf, &c. are defcribed under ELECTRICITY. The better a conductor that any fubftance is, the greater is the difficulty of making the electric spark vifible in it. The rarity of any body greatly increa fes the ease with which the electric fpark is made vifible in it; as appears from difcharging a vial through rarefied air, the vapour of ether, ipirit of wine, or water. In the profecution of his experiments, our author cemented a ball of iron into the orifice of a tube 48 inches long, and two thirds of an inch diameter, fo that it could bear the weight of the quickfilver with which the tube was filled all to a small space at the open end, which contained a few drops of water. Having inverted the tube, and plunged the open end of it into a bafon of mercury, that in the tube stood nearly half an inch lower than in a barometer with which it was compared at the fame time, owing to the vapour which was formed by the water; but the spark passed as brilliant through the rarefied water as it does through rarefied air. If fpirit of wine be employed inftead of water in this experiment, the fpark will not be fo luminous. In the vapour of ether a great force is requifite to make the fpark luminous, but good ether will prefs the mercury down as far as 16 or 17 inches. By rarefying the vapour, however, the fpark will pass through it with more eafe. On examining the mineral acids in vacuo, Mr Morgan could not find that any vapour escaped from them. To give them the requifite degree of tenuity, therefore, he traced a line upon glafs about an eighth part of an inch broad, with a camel's hair pencil dipped in the acids: the line extended fometimes to

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the length of 27 inches; in which cafe, the electric fpark would pafs over the whole with great brilliancy. If by widening the line, however, or putting on a drop of acid in any particular part, the quantity was increased, the fpark never appeared in that part. The brightnefs of the electric light is always in proportion to its condenfation. Thus, if a fpark taken from a powerful electrical machine divides itself into brushes, or throws out fparks from the fides, by which the light is diffused over a larger furface, it thus becomes lefs brilliant; and in all cafes in which any diffufion of light, whether electric or not, takes place, the cafe will be the fame. In fome cafes, Mr. Morgan is of opinion, that even with the electric fluid, only the more refrangible rays of light make their efcape. Thus, the electrical brush is always of a purplish or bluish colour; and if you convey a spark through a Torricellian vacuum not very perfectly made, it will be of an indigo colour. This, however, does. not seem to arife from any other caufe than the mere weakness of the light, which, in paffing through the vapours of the atmosphere, or perhaps through the humours of the eye itself, affects our organs of fight in that manner. He next examines the influence of media upon electric light; which, he fays, is fimilar to their influence upon folar light, and serves to explain feveral phenomea. "Let a pointed wire (fays he), having a metallic ball fixed to one of its extremities, be forced obliquely into a piece of wood, fo as to make a fmall angle with its furface, and to make the point lie about one eighth of an inch below it. Let another pointed wire, which communicates with the ground, be forced in the fame manner into the fame wood, fo that its point may in like manner be about of an inch below the furface, and about two inches from the point of the firft wire. Let the wood be infulated, and a ftrong fpark which strikes on the metallic ball will force its way through the interval of wood which lies between the points, and appear as red as blood." Mr Morgan mentions fome experiments which feem to militate against his hypothefis rather than to fupport it; viz. 1. If into a Torricellian vacuum of any length, a few drops of ether are conveyed, and both ends of the vacuum ftopped up with me tallic conductors, so that a spark may pass through it, the spark in its paffage will make the following appearances. When the eye is placed clofe to the tube, the fpark will appear perfectly white; if the eye is removed 2 yards, it will appear green; but at the distance of 6 or 7 yards, it will appear reddish. "Thefe changes evidently depend (fays Our author) on the quantity of medium through which the light paffes; and the red light more particularly, which we fee at the greateft diftance from the tube, is accounted for on the fame principle as the red light of the beclouded fun, or lighted candle." 2. Dr Priestley long ago obferved the red appearance of the electric fpark, when paffing through inflammable air. But this appearance is very much diverfified according to the quantity of medium through which the fpark is beheld. At a very confiderable diftance the red comes unmixed to the eye; but if the eye be laced close to the tube, the spark appears white

and brilliant. By increasing, however, the quantity of fluid conveyed through any portion of inflammable air, or by condenfing that air, the fpark may be made perfectly white. All weak explosions and fparks viewed at a distance, make a reddish appearance. The reafon seems to be, that the weaker the spark or explosion is, the more it is disposed to assume a red colour when viewed at a diftance. This feems to confirm what has already been mentioned as a probable hypothefis, that the different colours of light are entirely ow ing to the medium through which they are viewed. (9.) LIGHT, MR MORGAN'S EXPERIMENTS ON PHOSPHORIC. On phosphoric light Mr Morgan makes fome curious obfervations; but ftill argues on the fame principles: "Some fhells (fays he), prepared according to Mr Wilson's directions, (fee PHOSPHORUS), after being exposed to the fun or to the flash of a battery, emit a purple, others a green, and others a reddish light. If, with Mr Wilson, we fuppofe that these fhells are in a state of flow combustion, may we not conclude that fome are just beginning to burn, and therefore emitting the most refrangible rays: while others are in a more advanced state of combustion, and therefore emitting the leaft refrangible? If this conclufion be right, the fhells which are emitting the purple or green, muft ftill retain the yellow, the orange, and the red, which will allo make their appearance as foon as the combuftion is fufficiently increased." In confirmation of this, Mr Morgan adduces the following experiment, viz. that if a fhell, while emitting its green rays, be placed upon a warm shovel, the colour will foon be changed into a yellow mixed with red. To the theory of flow combuftion Mr Morgan objects, 1." If phosphoric fhells owe their light to this caufe, we muft confider the word coмBUSTION, when applied to them, as implying all thofe circumftances which usually attend a body when on fire. On this fuppofition there ought to be an increase of the heat as well as of the decompofition of the combuftible. But neither of thefe take place, for a phosphoric body never fails to lose its light entirely in a certain degree of heat, without lofing the power of becoming phosphoric again when it has been fufficiently cooled. While very hot, the charge of the ftrongest battery conveyed over it has no effect. 2. When bodies are wafted by combuftion, they can never be made to reaffume the appearances which they previously difplayed. "No power (fays he) can give to afhes the phenomena of a burning coal. But phosphoric bodies are very different in this refpect; for a phosphoric fhell may be made to lofe all its light by expofure to heat, and again may be made as luminous as ever by exposure to the fun." Some bodies which are most beautifully phosphoric, are at the fame time the moft obftinate in refifting fire. "Let us now fee (fays Mr Morgan) the confequence of admitting the common hypothefis, that the detention of thofe rays which fall upon phosphori is owing to fome force which prevents their immediate reflection, but is not ade. quate to their entire abforption. This force, whatever it be, cannot well be fuppofed to operate with equal power on all thefe rays. If this be not the cafe, we cannot avoid concluding, that phosphoric

39

ing, has not been afcertained. We are altogether ignorant of the component parts of every one of thefe fpecies. The 2d peculiar property of light is the prodigious velocity with which it moves, whenever it is feparated from any body with which it was formerly combined." (See § 4, 5.) "This velocity it acquires in a moment, and in all cafes, whatever the body be from which it feparates. The 3d and not the leaft fingular of its peculiar properties, is, that its particles are never found cohering together fo as to form masses of any fenfible magnitude. This difference between light and other bodies can only be accounted for, by fuppofing that its particles repel each other. This feems to conftitute the grand diftinction between light and" other "bodies. Its particles repel each other, while the particles of other bodies attract each other; and, accordingly, are found cohering together, in maffes of more or lefs magnitude." Syft. of Chem. vol. I. p. 252, 253.

phofphoric fhells will affume different colours, owing to the earlier and later escape of the diffe. rent rays of light. This conclufion is justified by an experiment already mentioned; viz. that when the force is fuch as to admit the escape of the purple, blue, and green, we have only to lessen that force, by warming the body, and the yellow, the orange, and red efcape. Beccaria has proved, that there is scarcely any body which is not phof. phoric, or may not become fo by heat. But as the phofphoric force is moft powerful when the purple rays only efcape, fo we are to conclude, that it is weakest when it is able to retain the red rays only. This is agreeable to several facts. Chalk, oyster-fhells, together with those phosphoric bodies whofe goodness has been very much impaired by long keeping, when finely powdered, and placed within the circuit of an electrical battery, will exhibit, by their fcattered particles, a hower of light; but these particles will appear reddish, or their phosphoric power will be fufficient only to detain the yellow, orange, and red. rays. When fpirit of wine is in a similar manner brought within the circuit of a battery, a fimilar effect may be discovered: its particles diverge in several directions, displaying a most beautiful golden appearance. The metallic calces are render, ed phosphoric with the greateft difficulty; but even these may be scattered into a shower of red luminous particles by the electric ftroke." In a P. S. to this paper by Dr Price, it is obferved, that by phosphoric force, Mr Morgan feems to mean, not the force with which a phosphoric body emits, but that with which it absorbs and retains, the light, This laft force is proportioned to the degree of attraction between the phofphoric body and light; and therefore muft, according to Mr Morgan's theory, be weakest when it fo freely emits the light it has imbibed as not to retain those rays which adhere to it most strongly. According to Mr Morgan's theory, thefe are the rays which are the leaft refrangible. "It is, however, (fays Dr Price), an objection to it, that the lefs refrangibility of rays feems to imply a lefs force of attraction between them and the fubftances which refract them; but it fhould be confidered, that, poffibly, the force of cohefion, which unites the rays of light to bodies, may be a different power from that which refracts them."

(10.) LIGHT, PECULIAR PROPERTIES OF. Dr T. Thomson, above quoted, (§ 4.) after enumerating the general properties of light mentioned above, and under CHEMISTRY, ELECTRICITY, &c. concludes thus: "Such are the properties of light as far as they have been examined. They are fufficient to convince us, that it is a body, and poffeffes many qualities in common with other bodies. It is attracted by them, and combines with them, precisely as other bodies do. But it is diftinguished from all" other "fubftances, by poffeffing three peculiar properties of which they are deftitute: The ift is the power which it has of exciting in us the fenfation of vifion, by moving from the object feen and entering the eye. The phenomena of colours," (See CHROMATICS, Sec. I.—III.)" and the prifmatic spectrum, indicate the existence of different fpecies of light; but to what the difference of thefe fpecies is ow

(11.) LIGHT, PHENOMENA RESPECTING, IN PLANTS Most of the difcous flowers, by fome power unknown, to us, follow the fun in his courfe. They attend him to his evening retreat, and meet his rifing luftre in the morning with the fame unerring law. If a plant is fhut up in a dark room, and a small hole is afterwards opened, by which the light of the fun may enter, the plant will turn towards that hole, and even alter its own fhape in order to get near it; fo that though it was fraight before, it will in time become crooked, that it may get near the light. It is not the heat but the light of the fun, which it thus covets; for, though a fire be kept in the room, capable of giving a much stronger heat than the fun, the plant will turn away from the fire to enjoy the fun's light. The green colour of plants alfo depends on the fun's light being allowed to fhine upon them; for without this they are always white.

(12.) LIGHT PROCEEDING FROM ANIMAL SUBSTANCES, &C. INDEPENDENT OF HEAT. In ge neral, a very confiderable degree of heat is requifite to the emiffion of light from any body; but there are feveral exceptions, especially in light proceeding from putrefcent fubftances and phofphorus, together with that of luminous animals, and other fimilar appearances. Light proceeding from putrefcent animal and vegetable fubftances, as well as from glow worms, is mentioned by Ariftotle. Thomas Bartholin mentions 4 kinds of luminous infects, two with and two without wings; but in hot climates they are found in much greater numbers, and of different fpecies. (See FIRE-FLIES and LAMPYRIS.) Columna, an induftrious naturalift, obferves, that their light is not extinguished immediately upon the death of the animal. The firft diftinct account that we meet with of light proceeding from putrefcent animal flesh, is that which is given by Fabricius ab Aquapendente; who fays, that when three Roman youth, refiding at Padua, had bought a lamb, and had eaten part of it on Eafter day 1592, feveral pieces of the remainder, which they kept till the day following, thone like so many candles when viewed in the dark. Part of this luminous flesh was fent to Aquapendente, profeffor of anatomy in that city. He obferved, that both the lean and the fat fhone with a whitish kind of

light;