A Letter from M. de Humboldt to M. Pictet, on the Magnetic Polarity of a Mountain of Serpentine .

I received this communication in manuſcript from the Right Hon. Sir Joſeph Banks, Bart. P. R. S. &c. It is written in French, under the title of “Lettre quatrième de M. de Humboldt à M. Pictet, ſur la Polarité Magnetique d’une Montagne de Serpentine.”

The ſame liberal promoter of ſcience has favoured me with a ſpecimen of this rock, with permiſſion to make experiments upon it. A few obſervations on this ſpecimen are added at the end of this memoir. N.

At the beginning of the eighteenth century the attention of natural philoſophers was entirely fixed on the phenomena of magnetiſm.

The progreſs which has ſince been made in the theory of electricity, and the preponderance which chemiſtry has acquired over all the other branches of natural hiſtory, have diminiſhed the intereſt with which enquiries into the nature of the magnetic fluid ought to have been purſued.

It is true that your celebrated countrymen Meſſrs. de Sauſſure and Prevôt have given vigour to this purſuit by diſcoveries worthy of their ſagacity: the firſt, by inventing an inſtrument capable of meaſuring the comparative intenſity of the magnetic forces in different regions of the globe; and the other, by reducing the laws of polarity to the ſimple laws of attraction.

But theſe diſcoveries have not afforded inducement ſufficient to lead philoſophers into a path ſo honourably explored.

The moſt valuable work on the origin of magnetic forces has been neglected, together with the calculations of the ingenious Coulomb, and his experiments with the balance of Torſion.

Having traverſed with the compaſs in my hand great part of the mountains of Europe, I became convinced that declinations cauſed by maſſes of iron in beds or in veins are infinitely leſs frequent than naturaliſts affirm.

The obſervations which Meſſrs. de Sauſſure and Trembley have made on the ſummit of Cramont , appear to me the more curious, as it ſtands alone, and preſents to our knowledge a very extended image of the dimenſions of magnetic ſpheres.

It is among the Alps of Sweden and Norway, thoſe northern regions which nature has enriched with an enormous depoſit of iron leſs oxided than in our country, that we were entitled to expect ſimilar phenomena.

Voyage dans les Alpes, T. I. p. 375.—T. II. p. 343.

I haſten to communicate to you a diſcovery I made in the month of November, and which appears to me of conſiderable importance in the progreſs of geology.

You are acquainted, Sir, with the laws and the harmony which I have obſerved in the direction and inclination of the primitive ſtrata, from the banks of the Mediterranean to thoſe of the Baltic Sea.

You have even condeſcended, jointly with our friend Dolomieu, to expreſs an intereſt with regard to this laborious undertaking; which, in more ſkilful hands than mine, would, I am well aſſured, throw great light on the conſtruction of the globe.

I traverſed the chain of mountains of the High Palatinate and the margraviate of Bayreuth; and I found, in the bottom of the Fichtelgebirge, between Munichberg and Goldcronach, an iſolated hill, which riſes to the elevation of fifty toiſes above the ſurrounding plain.

Its height above the level of the ſea may be eſtimated at two hundred and eighty, or three hundred, toiſes.

This hill extends in length from weſt to eaſt, and forms a pyramid extremely obtuſe.

The rocks which crown the ſummit or ridge are compoſed of ſerpentine of conſiderable purity, which, by its colour and foliated fracture, approaches in various parts to the chlorithſchiefer of Werner (ſchiſtous chlorite).

This ſerpentine is divided into ſtrata rather diſtinct, of which the inclination to the north-weſt preſents an angle between 60 and 65 degrees.

It repoſes on a foliated granite, mixed with hornblende; a mixture which we diſtinguiſh by the name of ſyenite.

I approached this ſerpentine with the compaſs, in order to determine more accurately the angle it formed with the meridian.

The magnetic needle was in a ſtate of continual agitation.

I advanced two ſteps farther, and beheld that the north pole was entirely turned to the ſouth.

I called two friends, Meſſrs. Godeking and Killinger, who aſſiſted me in my geological purſuits; and we were alike penetrated with that joy which the contemplation of intereſting phenomena produces in the minds of thinking men.

I ſhall not detain your attention by a full recital of our obſervations; but ſhall merely preſent the reſults, to which I may hereafter make additions, if my occupations ſhould not lead me from this part of Germany.

The action of this mountain of ſerpentine upon the magnet ſhews itſelf in a very curious manner.

The uncovered rocks which are ſeen on the northern ſlope, and thoſe on the declivity towards the ſouth, have poles directly oppoſite.

The former exhibit only ſouth poles, and the latter north poles.

The whole maſs of foliated ſerpentine does not therefore poſſeſs a ſingle magnetical axis, but preſents an infinity of different axes perfectly parallel to each other.

This paralleliſm alſo agrees with the magnetic axis of the globe, though the poles of the ſerpentine are inverted; ſo that the northern pole of the hill is oppoſed to the ſouth pole of the earth.

The eaſt and weſtern ſlopes preſent what in the theory of magnetiſm would be called points of indifference.

The magnet does not at this part appear to be in any reſpect affected, though the ſubſtance of the rock differs in no external character from the other parts.

It is the ſame on the ſouth ſide of the ſummit . I have obſerved not only that the magnetic axes are not diſpoſed in the ſame horizontal plane; but I have likewiſe remarked, that two points, of which the action is very ſtrong, are joined by rocks which do not exert the leaſt attraction.

The chemical analyſis of theſe compounds affords the ſame reſults; and it would be no leſs difficult to diſcover any difference of aggregation between them, than between iron which has received the touch, and other iron which had never acquired the magnetic power.

I ſuſpect an error of the copyiſt, as the words “Il en eſt de même du côté meridional de la ſommité contradict the general deſcription immediately preceding. N.

On this occaſion a queſtion preſents itſelf which cannot be reſolved in leſs than half a century.

The tables founded on the obſervations of Picard, La Hire, Maraldi, Caſſini, and Le Monnier, ſhew that the needle has declined ſince 1660 towards the weſt; and that this declination continues to increaſe, though the oſcillations cauſed by the heats of the ſouth, and the temperature of the ſeaſons, often produce a retrograde courſe.

If the magnetic axis of our mountain were aſtronomically determined by the culmination of the ſtars, whether its direction would remain the ſame until the year 1850, or whether its ſouth-pole would turn towards the weſt, in connection with the variation of the magnetic needle?

From our profound ignorance of the cauſes of the declination, as well as of moſt geological phenomena, it is not in our power to reſolve ſo complicated a problem.

Other obſervations equally intereſting may be made on the identity of magnetic forces.

I have diſcovered a maſs of rocks which affect the needle at the diſtance of twenty-two feet.

With an apparatus ſimilar to the magnetometer of M. de Sauſſure, we might obſerve whether the intenſity of the forces of magnetic action remains the ſame in winter and in ſummer; whether it be ſtronger in the morning, at noon, at the ſolſtices, during the aurora borealis, or in an atmoſphere loaded with electric fluid?

I ſuppoſe that theſe same rocks might act on the needle ſometimes at 16, and ſometimes at 28 feet diſtance.

It has been obſerved, that metals expoſed to the air gradually imbibe the magnetic fluid.

A ſlight oxidation of the iron ſeems to favour this effect.

I have myſelf obſerved, that in a magnetic bed of iron thoſe parts only which were in contact with the air affected the needle.

This phenomenon is conſidered as the effect of atmoſpheric electricity.

I am aware that lightning converts a bar of iron into a magnet; that the diſcharge of the Leyden vial ſometimes increaſes the intenſity of magnetic forces; but I do not ſee why the atmoſpheric electricity ſhould act ſimply on the external ſurface of a bed of magnetic iron, which is a good conductor of the electric fluid.

Does not the oxygene of the atmoſphere rather act a part in this operation?

Without wandering in the ſphere of probabilities, I have choſen to adhere to enquiries reſpecting facts.

I have obſerved the rocks which were covered with turf, from which I detached pieces that had not been in contact with the air. I found that the magnetic force was conſtantly the ſame.

The mountains of the Harz preſent a granite rock called the ſchnarcher, which is elevated in the form of a tower, or broken pyramid.

This granite likewiſe affects the needle; but it acts only in the maſs, and in a ſingle band or perpendicular vein.

Detached pieces ſhew no action upon the needle.

It is to Mr.

de Trebra, celebrated for his reſearches concerning the internal parts of mountains, that we are indebted for this important diſcovery.

Some philoſophers pretend that the ſchnarcher contain in their bowels a maſs of magnetic iron; others preſume that a ſtroke of lightning has cauſed the magnetic vein in theſe mountains.

The nature of the rocks which I have the honour to preſent to your notice in this paper does not admit of ſimilar explanations.

The ſerpentine not only acts in a maſs, in its natural ſituation, but all the pieces, when broken, to infinity, ſtill exhibit two very diſtinct poles.

Pieces of five inches diameter act on the needle at the diſtance of half a foot.

The examination of the magnetic axes affords an object of curious enquiry.

They are moſtly found in a direction parallel to that of the foliated grain; nevertheleſs, I have found ſome which croſs it perpendicularly.

Fragments extremely ſmall, of the magnitude of 0.01 of a cubic line, ſhew a very ſtrong polarity in proportion to their maſſes.

You ſee them turn very ſuddenly when the poles of the weakeſt magnet are ſucceſſively preſented to them. It is a very ſtriking phenomenon, that a ſtone poſſeſſed of ſo high a degree of polarity ſhould exhibit no attraction for iron which is not magnetiſed.

I have never obſerved the ſmalleſt particle of filings of iron adhere to the ſerpentine; but the ſerpentine, reduced to powder, attaches itſelf very readily to the magnet.

You will enquire with impatience, if it be well proved that my ſerpentine is not mixed with magnetic iron; whether this mixture may not be ſufficiently intimate to enter into the compoſition of each particle of the rock?

I can aſſure you, that I have made the moſt aſſiduous enquiries in this reſpect.

All my experiments were made in conjunction with Mr. Godeking, whoſe knowledge and abilities are a ſufficient aſſurance againſt error; but we were decidedly convinced, that if the magnetic force cannot adhere to the earthy ſubſtances which form the baſe of the ſerpentine, it can be attributed only to the oxide of iron with which it is coloured.

Theſe are our reaſons:

The rock has no mixture of metallic ſubſtances.

It preſents only here and there a few fragments of tale or amianthus; but neither pyrites, nor ſchoerl, nor octahedrons of magnetic iron.

When reduced to very ſine powder, it reſembles pounded chalk.

The microſcope diſcovers only earthy parts, of a clear whitiſh green.

The ſpecific gravity of this ſerpentine is very ſmall.

I find it only from 1.901 to 2.04 aſſuming water to be 1.0.

There are not conſequently any minerals but pumice-ſtone, mountain-leather, and ſome varieties of the opal, which do not equal our ſerpentine in denſity.

The chemical experiments we have hitherto made, prove that it contains, like the jade or lapis ollaris, oxided iron; but not iron capable of attraction by the magnet.

The ſolutions in muriatic acid, mixed with the nitric acid, are yellow, and not green like thoſe which the micaceous iron, and all the ores which contain pure or metallic iron, afford.

Here then is a very ſtriking phenomenon, namely, the polarity of ſuper-oxigenated iron.

We learn by the valuable experiments of my celebrated countrymen Klaproth and Wenzel, that pure nickel and cobalt are attracted by the magnet; we know that iron ſlightly oxided (the black oxide) is alſo affected; but how great the difference between this ſtate of oxidation, and that of the iron which colours the ſerpentine, various calcareous ſtones, and perhaps even certain vegetable matters!

What difference between a ſubſtance which acts alike on the two extremities of the needle, and a ſtone of which the ſmalleſt portions exerciſe a ſpontaneous polarity!

Let us purſue the path of obſervation; let us collect indubitable facts.

By this method the theories of natural philoſophy will be eſtabliſhed on ſolid and durable foundations.