Imágenes de páginas
PDF
EPUB

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 reapproach 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.

Iothly. 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. Æpinus, for this hypo

thesis, 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.

CCCCXCI

TO M. DUBOURG

10 March, 1773.

I shall not attempt to explain why damp clothes occasion colds, rather than wet ones, because I doubt the fact; I imagine that neither the one nor the other contribute to this effect, and that the causes of colds are totally independent of wet and even of cold. I propose writing a short paper on this subject, the first moment of leisure I have at my disposal. In the meantime I can only say that, having

some suspicions that the common notion which, attributes cold to the property of stopping the pores and obstructing perspiration, was ill founded, I engaged a young physician, who is making some experiments with Sanctorius' balance, to estimate the different proportions of his perspirations, when remaining one hour quite naked, and another warmly clothed. He pursued the experiment in this alternate manner for eight hours successively, and found his perspiration almost doubled during those hours in which he was naked.

B. FRANKLIN.

CCCCXCII

TO WILLIAM FRANKLIN

LONDON, 15 March, 1773.

DEAR SON: I wrote you pretty largely by Capt. All, and sent you sundry things, particularly the plated boiler you wrote for. I have nothing to add, but to let you know I continue well. Enclosed I send you the Boston pamphlet with my preface. I grow tired of my situation here, and really think of returning in the fall. My love to Betsey.

I am ever your affectionate father,

B. FRANKLIN.

CCCCXCIII

TO ABEL JAMES AND BENJAMIN MORGAN LONDON, 15 March, 1773. GENTLEMEN:-In mine of February 10th, I mentioned a silk weaver who was desirous of going to

America; and endeavoring to get subscriptions among his friends to defray the expense of his and family's passage. He now tells me they have been so kind as to double the sum he requested, and that he is to go in Sutton. He takes with him a good certificate from the meeting; and I beg leave to recommend him to the notice and encouragement of the silk committee, as far as they may find him deserving. For though it may be most advantageous to our country, while the bounty continues so high, to send all our raw silk hither, yet as the bounty will gradually diminish and at length cease, I should think it not amiss to begin early the laying a foundation for the future manufacture of it; and perhaps this person, if he finds employment, may be a means of raising hands for that purpose. His name is Joseph Clark.

By the enclosed you will see when the silk will probably be sold. I hope to send you a good account of it, and am, with great esteem, gentlemen, your most obedient, humble servant,

B. FRANKLIN.

CCCCXCIV

FROM M. DUBOURG

PARIS, 25 March, 1773.

SIR: 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 endeavor 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 earthly solvents; for there are different

« AnteriorContinuar »