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other, it is not easy for it to return; and thus a bar of steel is converted into a permanent magnet.

6thly. A great heat, by expanding the substance of this steel, and increasing the distance between its particles, affords a passage to the magnetic fluid, which is thus again restored to its proper equilibrium; the bar appearing no longer to possess magnetic virtue.

7thly. A bar of steel, which is not magnetic, being placed in the same position, relatively to the pole of the earth, which the magnetic needle assumes, and in this position being heated and suddenly cooled, becomes a permanent magnet. The reason is, that while the bar was hot, the magnetic fluid which it naturally contained was easily forced from one extremity to the other by the magnetic virtue of the earth; and that the hardness and condensation, produced by the sudden cooling of the bar, retained it in this state without permitting it to resume its original situation.

8thly. The violent vibrations of the particles of a steel bar, when forcibly struck in the same position, separate the particles in such a manner during their vibration, that they permit a portion of the magnetic fluid to pass, influenced by the natural magnetism of the earth; and it is afterwards so forcibly retained by the re-approach of the particles, when the vibration ceases, that the bar becomes a permanent magnet.

9thly. An electric shock passing through a needle in a like position, and dilating it for an instant, renders it, for the same reason, a permanent magnet; that is, not by imparting magnetism to it, but by allowing its proper magnetic fluid to put itself in motion.

10thly. Thus there is not in reality more magnetism in a given piece of steel after it is become magnetic, than existed in it before. The natural quantity is only displaced or repelled. Hence it follows, that a strong

apparatus of magnets may charge millions of bars of steel, without communicating to them any part of its proper magnetism; only putting in motion the magnetism which already existed in these bars.

I am chiefly indebted to that excellent philosopher of Petersburg, Mr. Epinus, for this hypothesis, which appears to me equally ingenious and solid. I say chiefly, because, as it is many years since I read his book, which I have left in America, it may happen, that I may have added to or altered it in some respect; and, if I have misrepresented any thing, the error ought to be charged to my account.

If this hypothesis appears admissible, it will serve as an answer to the greater part of your questions. I have only one remark to add, which is, that, however great the force is of magnetism employed, you can only convert a given portion of steel into a magnet of a force proportioned to its capacity of retaining its magnetic fluid in the new position in which it is placed, without letting it return. Now this power is different in different kinds of steel, but limited in all kinds whatever. B. FRANKLIN.

FROM M. DUBOURG TO B. FRANKLIN.

On the Choice of Glass for the Leyden Experiment.

SIR,

Paris, 25 March, 1773.

If I have rightly understood your principles, the glass to be used in the Leyden experiment ought to combine these two qualities; first, it should be impermeable to the electric fluid; secondly, it should not be impermeable to the action of this fluid; or, to express the same

thing in other words, the electric fluid must not be able to pass from one surface to the other, but its afflux on one of the surfaces of the glass must have the power to excite an efflux on the opposite surface.

Glass generally unites these two qualities, but not every kind of glass. There is even glass that the electric fluid passes through, almost as readily as it enters metals. This is a property natural. to some kinds of glass, and accidental to others. It would seem astonishing, that no philosopher had yet thought of seeking out the causes of all these differences, if natural philosophy alone were equal to the task; but there is need of the aid of chemistry, which certainly may throw some light on so interesting a subject.

I would not propose to the chemists to analyze the different kinds of glass, permeable or impermeable to electricity; but to endeavour to imitate them, which would be much easier for them to do.

Pure vitrifiable earth is without doubt the only ingredient in rock crystal, which may be considered as a true natural glass; but art has not yet succeeded in obtaining for us a glass so pure, and there is even very little reason to hope that such perfection can ever be attained.

There is no earth known so vitrifiable as not to require some auxiliary solvent to facilitate its vitrification. Now solvents are distinguished into three principal kinds, which are, saline solvents, metallic solvents, and earthy solvents; for there are different kinds of earths, which, although each singly is refractory, yet serve as mutual solvents, as there are also many kinds of salts, and many kinds of metals, which may be used as solvents for the vitrifiable earths, and which may be combined in different proportions with the same earths. We ought not to be more surprised to find glass more or

less permeable to electricity, than to find it pervious and impervious to light. Since there is transparent glass and opaque glass, or glass of various colors, why should there not be glass which is a conductor, and that which is a non-conductor, of electricity?

It would not be a problem of difficult solution for a chemist, but yet it would be a labor requiring considerable time, to furnish us with a comparative table of the different kinds of glass possessing either of these qualities in all their various degrees. The places merely, occupied by your greenish American glass, as well as by the white London glass, would indicate, at the first glance, the mixture of ingredients of which they are respectively composed.

. On the other hand, as the intensity of heat to which the substance of the glass is exposed, whether in 'melting or annealing, may cause the evaporation of some of these ingredients, and as this heat is not equally powerful in every part of the furnace, it is not very surprising, that you should have found considerable difference between several glass globes from the same manufactory, as you inform us.

Independently of the natural properties of one kind of glass or another, arising from their particular composition, great differences may also result from the different thickness of their masses, were it from this consideration alone, that the heat could not be precisely the same, nor the rapidity of cooling very nearly equal, in the different layers of very thick glass; without taking into the account, that it seems almost impossible, that the action of the electric fluid in motion should be effectually conveyed from one surface to another of a very massive body.

Lastly; it is equally easy to conceive, that a considerable degree of heat, by rarefying the substance of thin

glass, should open its pores to the electric fluid; but that the degree of heat must be in proportion to the thickness of the glass; and that Mr. Kinnersley found a heat of only two hundred and ten degrees (the point at which water boils, according to Fahrenheit's thermometer,) necessary to render the very thin glass of a Florence flask permeable to the electric shock, while Mr. Cavendish required a heat of four hundred degrees to make glass a little thicker permeable to the common

stream.

My reason for wishing that some chemist would have the goodness to enlighten us upon all these points is, that too much pains cannot be taken to spare the lovers of natural philosophy any unnecessary expense; because this may turn some entirely aside from its pursuit, and somewhat damp the zeal of many others. I am, &c.

*

TO MESSRS. DUBOURG AND DALIBARD.*

Concerning the Mode of rendering Meat tender by Electricity.

MY DEAR FRIends,

My answer to your questions, concerning the mode of rendering meat tender by electricity, can only be founded upon conjecture; for I have not experiments enough to warrant the facts. All that I can say at present is, that I think electricity might be employed for this purpose, and I shall state what follows as the observations or reasons which make me presume so.

This letter has no date, but the one to which it is an answer is dated May 1st, 1773.

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