pole to the other internally, and is then carried back in curve lines externally, till it arrives again at the pole where it first entered, to be again admitted.

Exper. 1.-If we lay a magnetical body under a piece of paper or glass, that is strewed over with steel filings or magnetical sand, and by striking the table put the filings in motion, they will readily dispose themselves in such a manner as to represent, with great exactness, the course of the magnetic matter. Steel rendered magnetical is best for this purpose, because it is of a more uniform texture than loadstones, and will on that account exhibit a more regular appearance. By this experiment the curve lines in which the magnetical matter returns back to the pole where it first entered are accurately expressed by the arrangement of the filings. The largest curves are such as take their rise from one polar surface, and are extended to the other; being larger in proportion as they arise nearer the axis or centre of the polar surface. Those curves which arise from the sides of a magnetical body are always interior to those which arise from the polar surface; and are less and less in proportion to their distance from the ends. If any one should doubt whether the magnetical matter, which thus disposes the filings, is really moving back in a direction contrary to that with which it passes through the magnetical body, let him try it in different parts with a small compass needle, and the fact will appear beyond dispute.

Exper. 2.-The larger the distance is from pole to pole in different magnets, the larger will these curves be. This appears from examining magnets of different lengths. And this is the reason why, in the same magnet, the curves are less in proportion to their greater distance from the ends of the bars. For the poles from whence these curves arise are proportionably nearer each other.

Exper. 3.-If the south pole of one magnet be opposed to the north of another, most of the magnetic matter is carried directly out of one into the other: and does not return back in curve lines till after having passed through both magnets. It appears from the arrangement of the filings, that the magnetic matter proceeding from the polar surface, does not now diverge from the axis as before, but runs more in straight lines, till it arrives at the polar surface of the other magnet. The curves arising from the sides, which before were bent towards the opposite end of the same magnet, are many of them now bent the contrary way, towards the corresponding sides of the other magnet. Those which are not bent the contrary way, are such as are too remote from the opposed pole of the other magnet to be influenced by it; and therefore continue their natural course.

Exper. 4.-While the bars are in the position of the last experiment, if a small loadstone be placed in the stream running from one to the other, in any position whatever, the stream will pass through the stone: which, being again removed, will be found to have a polarity exactly in the direction of that stream.

Exper. 5.-If the north or south poles of two magnets be opposed to each other, the filings will exhibit the appearance of two streams meeting; and the curves of each will all be turned towards the opposite pole of the same magnet. The appearance is altogether the same, whether the two north or two south poles be opposed to each other. So that it is not to be determined, from any of these experiments, at which of the poles the magnetic stream enters. As we have some reason to think it enters at the north pole, we may suppose that to be the case, without danger of error; provided we build nothing on the supposition, but what would hold good, mutatis mutandis, if the contrary should be true. This being supposed, when the south poles are opposite, the two streams coming out at them are directly contrary, by which the magnetic matter is accumulated, and therefore diverges so much the faster to return back to the north poles. When the north poles are opposed to each other, the streams of magnetic matter returning from the south poles are directly contrary; and, by crowding at once towards each polar surface, are accumulated between them, and converge towards them so much the faster.

These 5 experiments seem sufficient to establish the truth of the proposition; and many more might be produced to the same purpose.

PROP. 2.-The immediate cause why two or more magnetical bodies attract each other, is the flux of one and the same stream of magnetical matter through them.

Exper. 6.—It appeared in the 3d experiment, that when the south pole of one magnet was opposed to the north of another, a stream of magnetic matter was carried from one to the other, and did not return back to the pole where it first entered, till after having passed through both bars: and it is needless to observe that two bars in this position are in a state of attraction. The 5th experiment showed, that when the two south or north poles were opposed, there was no stream common to both. Now it is well known, that magnetical bodies in this situation are so far from attracting, that they strongly repel each other. If the 3d experiment be repeated, with the magnets placed at different distances from each other, we shall find that more of the magnetical matter will pass from one polar surface to the other, in proportion as the distance between is less. The attraction is therefore greater as the distances diminish. And at distances where none of the magnetic stream passes from one magnet to the other, there is no sign of attraction. So that this cause is not only co-existent with the effect, but also proportionable to it.

Exper. 7.-If a piece of soft iron, which has no fixed magnetism, be any where placed in the magnetical stream, it will be in a state of attraction while it remains in that stream, and no longer.

Exper. 8.-A ball of soft iron, in contact with the pole of a magnet, will at

tract a 2d ball, and that a third, and so on, till the stream become too weak to produce an attraction sufficient to support a greater weight.

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Exper. 9.-Having hung a number of balls to each other, by applying the first to the north pole of a magnet, on presenting the south of another magnet to one of the middle balls; all those below it will be deprived of the magnetic stream, and instantly losing their power of attraction fall asunder: the ball to which the magnet was applied will be attracted by it, aud all the others will still remain suspended. But if the north end of a magnet be presented, then the ball to which it is applied will also drop.

Exper. 10. In a magnet unarmed, the magnetic stream is carried back on all sides in curve lines to the contrary pole, as was seen in Exper. 1; but when armour is applied to each pole, the magnetic matter is conducted to the feet of the armour; and a lifter being thus applied to the feet, the whole stream coming out at one pole is carried back through it to the other; by which means the lifter is made to adhere to the feet of the armour with very great force. When the lifter is thus in contact, the magnet seems externally to have lost the greatest part of its force; though in reality it never acted with more. If instead of the lifter, we suspend a number of iron balls in contact, they will adhere together, and hang like a bracelet between the two feet; the returning stream passing now through them, as before through the lifter. Present the pole of a magnet, and they instantly fall asunder.

PROP. 3.-The immediate cause of magnetic repulsion, is the conflux and accumulation of the magnetic matter.

It appeared in the 5th Exper. that the same poles of 2 different magnets being opposed to each other, there was a conflux and accumulation of the magnetic matter; and we find by experience that all magnetical bodies in a like situation are in a state of repulsion.

Exper. 11.-Two small bars, the one hard, the other of a spring temper, being both magnetical matter, were opposed to each other, south to south; the filings produced the same appearance of repulsion, as described in the 5th experiment; then the bars being brought so near as to touch each other at the same poles, the repulsion was instantly changed into attraction.

On the Usefulness of Thermometers in Chemical Experiments; and concerning the Principles on which the Thermometers now in Use have been Constructed; with the Description and Uses of a Metalline Thermometer. Newly Invented by Cromwell Mortimer, M.D., Sec. R. S. &c. p. 672. Appendix to No 484. Chemistry, being the most extensive branch of experimental philosophy, has furnished mankind with the greatest number of curious and useful discoveries: for not only the art of separating metals from their ores, of which metals are

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formed such variety of useful instruments, but likewise cookery, which is so much concerned about the food of mankind during health, and also pharmacy, which furnishes medicines for restoring health when lost, the art of dying, and many other useful manufactures, all owe their improvements to this science; many of which have occurred unexpectedly to the operator while he had something else in view: but in many cases the chemists complain, that, having once accidentally hit on a curious experiment, on endeavouring to repeat it, they have never been able to make their process succeed exactly, as it did the first time, though they made use of the same materials, in the same quantity, and conducted the process through exactly the same operations. Where then must the cause of the miscarriage lie? Surely in the degree of heat made use of in the two experiments; for in many common operations, how usual is it for a preparation to be spoiled either by too little, or most commonly by too much fire, too long or too short a time applied! In order therefore to prevent these many miscarriages, Dr. M. advises the chemist, in his operations, to observe his clock with as much exactness as the astronomer does in his observations; and in order to know to a certainty the very degrees of heat he ever made use of in any process, that so he may be able to repeat and continue the same again in any repetitions of the same experiment. Let him have his laboratory furnished with various kinds of thermometers, proportioned to the degree of heat he intends to make use of. He will find these instruments as useful to him in his processes, as they have proved to the curious gardener in his stoves, who by them is taught to keep his plants in the same degrees of heat as are natural to them in their respective climates; which has been set forth in tables, after a very ingenious manner, by Mr. Sheldrake of Norwich. And besides enabling him to perform his operations with more exactness, these instruments would save him a great deal of fuel; for as liquors while boiling are not capable of receiving a greater degree of heat, all fuel which is used more than to keep them in that state, is useless; and the like happens in many other cases.

These instruments would also be of great service to maltsters, brewers, distillers, and vinegar-makers; for, by thermometers placed in different parts of the heap of wetted malt, the proper heat for its sprouting might be determined, and then regulated: the same for the heat of the kiln when the malt is spread on it. By thermometers the brewer may ascertain the heat of the water when he pours it on the malt, the heat of the wort when he sets it to work, and the heat while working: and in like manner the distiller and vinegar-maker, in short, every artificer who employs heat in his business, may by these instruments be certain of every degree necessary in each part of his work.

Many experiments show that all known bodies, whether fluid or solid, increase their bulk, or rarefy, by an addition of heat; and on the contrary, contractor

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become more dense by the diminution of heat, which is the presence of cold: and these alterations are always more or less sensible as the bodies are more or less dense.

The air we live in, as it is the most rare and light fluid, so are its alterations the most sensible; and indeed he knows of no experiments which determine how far it is capable of being expanded by heat, or condensed by cold; only we find that it will make its way through any fluid in which it lay dormant, when its elastic property is rouzed by the approach of such a heat as will make the fluid boil. On the other hand, when compressed by a fluid so contracted by cold as to freeze, or become solid, its elasticity will only bear a certain degree of compression, till the force with which it endeavours to restore itself, exceeds the force by which the parts of the solid, that confines it, adhere to each other, and so bursts its prison; as we often see during hard frosts in ice, and likewise glass, and other hard bodies, whose parts cannot stretch.

Next to air is alcohol, or the highest rectified spirits of wine: this and water, and all other liquids, are capable of receiving no greater degree of heat than what makes them boil, as was first demonstrated by M. Amontons; but that ingenious inventor of the quicksilver thermometer, Mr. Fahrenheit, has discovered, that when the barometer marks a greater pressure of the atmosphere, the same liquor will receive 8 or 9 degrees more of heat, than when the barometer is at the lowest. Hence Boerhaave gives the hint, that from nice experiments being made of the different degrees of heat, marked by a thermometer in boiling water, compared with the different heights of the barometer, and tables formed on them, a thermometer applied to boiling water might, at sea, where the motion of the ship hinders observations with the barometer, serve to determine the difference in the gravity of the atmosphere.

These, and all other liquids, by a certain determinate degree of cold peculiar to each sort, lose their fluidity, and freeze, or become solid, but not in the same order as by heat they boil; for, by cold, oil or water is sooner frozen than spirit of wine, though this will boil sooner than oil or water. All solid bodies likewise, as minerals, metals, and even stones, will become fluid, or melt, at a certain degree of heat peculiar to each species; and, when thoroughly melted, it is probable they are capable of receiving no higher degree of heat; and, on the absence of that heat to a certain degree, they all return to their natural solid state. Hence we may reasonably conclude, that solidity is the natural state of all bodies; and that some are only accidentally fluid, because their constitution is such as to melt by those degrees of heat which our atmosphere is most commonly subject to. All solid bodies are observed to contract into smaller dimensions by cold, and gradually to expand at the approach of heat, till at last, being by heat forced to the greatest degree of expansion, the particles of which they are com