Digitale Ausgabe

At the beginning of the eighteenth century the attention of natural philoſophers wasentirely fixed on the phenomena of magnetiſm. The progreſs which has ſince been madein the theory of electricity, and the preponderance which chemiſtry has acquired over allthe other branches of natural hiſtory, have diminiſhed the intereſt with which enquiriesinto the nature of the magnetic fluid ought to have been purſued. It is true that yourcelebrated countrymen Meſſrs. de Sauſſure and Prevôt have given vigour to this purſuit bydiſcoveries worthy of their ſagacity: the firſt, by inventing an inſtrument capable of mea-ſuring 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 theſediſcoveries have not afforded inducement ſufficient to lead philoſophers into a path ſo ho-nourably explored. The moſt valuable work on the origin of magnetic forces has beenneglected, together with the calculations of the ingenious Coulomb, and his experimentswith the balance of Torſion. Having traverſed with the compaſs in my hand great part of the mountains of Europe, Ibecame convinced that declinations cauſed by maſſes of iron in beds or in veins are in-finitely 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

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* 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 PolaritéMagnetique d’une Montagne de Serpentine.” The ſame liberal promoter of ſcience has favoured me with aſpecimen of this rock, with permiſſion to make experiments upon it. A few obſervations on this ſpecimen areadded at the end of this memoir. N. † Voyage dans les Alpes, T. I. p. 375.—T. II. p. 343.|98| it ſtands alone, and preſents to our knowledge a very extended image of the dimenſionsof magnetic ſpheres. It is among the Alps of Sweden and Norway, thoſe northern regionswhich nature has enriched with an enormous depoſit of iron leſs oxided than in our country,that we were entitled to expect ſimilar phenomena. I haſten to communicate to you a diſcovery I made in the month of November, andwhich appears to me of conſiderable importance in the progreſs of geology. You are ac-quainted, Sir, with the laws and the harmony which I have obſerved in the direction andinclination 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 anintereſt 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 traverſedthe 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 thelevel of the ſea may be eſtimated at two hundred and eighty, or three hundred, toiſes. Thishill extends in length from weſt to eaſt, and forms a pyramid extremely obtuſe. The rockswhich 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 chlorithſchieferof Werner (ſchiſtous chlorite). This ſerpentine is divided into ſtrata rather diſtinct, ofwhich the inclination to the north-weſt preſents an angle between 60 and 65 degrees. Itrepoſes on a foliated granite, mixed with hornblende; a mixture which we diſtinguiſh bythe name of ſyenite. I approached this ſerpentine with the compaſs, in order to determinemore accurately the angle it formed with the meridian. The magnetic needle was in aſtate of continual agitation. I advanced two ſteps farther, and beheld that the northpole was entirely turned to the ſouth. I called two friends, Meſſrs. Godeking and Kil-linger, who aſſiſted me in my geological purſuits; and we were alike penetrated with thatjoy which the contemplation of intereſting phenomena produces in the minds of thinkingmen. I ſhall not detain your attention by a full recital of our obſervations; but ſhallmerely preſent the reſults, to which I may hereafter make additions, if my occupationsſhould not lead me from this part of Germany. The action of this mountain of ſerpentine upon the magnet ſhews itſelf in a very curiousmanner. The uncovered rocks which are ſeen on the northern ſlope, and thoſe on thedeclivity towards the ſouth, have poles directly oppoſite. The former exhibit only ſouthpoles, and the latter north poles. The whole maſs of foliated ſerpentine does not thereforepoſſeſs a ſingle magnetical axis, but preſents an infinity of different axes perfectly parallel toeach other. This paralleliſm alſo agrees with the magnetic axis of the globe, though thepoles 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 mag-netiſm would be called points of indifference. The magnet does not at this part appearto be in any reſpect affected, though the ſubſtance of the rock differs in no external cha-racter from the other parts. It is the ſame on the ſouth ſide of the ſummit *. I have ob-ſerved not only that the magnetic axes are not diſpoſed in the ſame horizontal plane; but I

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* 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. |99| have likewiſe remarked, that two points, of which the action is very ſtrong, are joined byrocks which do not exert the leaſt attraction. The chemical analyſis of theſe compoundsaffords the ſame reſults; and it would be no leſs difficult to diſcover any difference of ag-gregation between them, than between iron which has received the touch, and other ironwhich had never acquired the magnetic power. On this occaſion a queſtion preſents itſelf which cannot be reſolved in leſs than half acentury. 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 thisdeclination 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 magneticaxis 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 geologicalphenomena, 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 whetherthe 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 acton 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 ina magnetic bed of iron thoſe parts only which were in contact with the air affected theneedle. This phenomenon is conſidered as the effect of atmoſpheric electricity. I amaware that lightning converts a bar of iron into a magnet; that the diſcharge of theLeyden vial ſometimes increaſes the intenſity of magnetic forces; but I do not ſee why theatmoſpheric 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 atmoſphererather act a part in this operation? Without wandering in the ſphere of probabilities, I havechoſen to adhere to enquiries reſpecting facts. I have obſerved the rocks which werecovered 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 elevatedin the form of a tower, or broken pyramid. This granite likewiſe affects the needle; but itacts only in the maſs, and in a ſingle band or perpendicular vein. Detached pieces ſhew noaction upon the needle. It is to Mr. de Trebra, celebrated for his reſearches concerningthe 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 magneticiron; others preſume that a ſtroke of lightning has cauſed the magnetic vein in theſemountains. The nature of the rocks which I have the honour to preſent to your notice in this paperdoes not admit of ſimilar explanations. The ſerpentine not only acts in a maſs, in its na- |100| tural ſituation, but all the pieces, when broken, to infinity, ſtill exhibit two very diſtinctpoles. Pieces of five inches diameter act on the needle at the diſtance of half a foot. Theexamination of the magnetic axes affords an object of curious enquiry. They are moſtlyfound in a direction parallel to that of the foliated grain; nevertheleſs, I have found ſomewhich croſs it perpendicularly. Fragments extremely ſmall, of the magnitude of 0.01 of acubic line, ſhew a very ſtrong polarity in proportion to their maſſes. You ſee them turnvery ſuddenly when the poles of the weakeſt magnet are ſucceſſively preſented to them. Itis a very ſtriking phenomenon, that a ſtone poſſeſſed of ſo high a degree of polarity ſhouldexhibit no attraction for iron which is not magnetiſed. I have never obſerved the ſmalleſtparticle 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 mixedwith magnetic iron; whether this mixture may not be ſufficiently intimate to enter into thecompoſition of each particle of the rock? I can aſſure you, that I have made the moſt aſſi-duous 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 wewere decidedly convinced, that if the magnetic force cannot adhere to the earthy ſub-ſtances which form the baſe of the ſerpentine, it can be attributed only to the oxide ofiron with which it is coloured. Theſe are our reaſons: The rock has no mixture of me-tallic ſubſtances. It preſents only here and there a few fragments of talc or amianthus; butneither pyrites, nor ſchoerl, nor octahedrons of magnetic iron. When reduced to very ſinepowder, it reſembles pounded chalk. The microſcope diſcovers only earthy parts, of a clearwhitiſh green. The ſpecific gravity of this ſerpentine is very ſmall. I find it only from1.901 to 2.04 aſſuming water to be 1.0. There are not conſequently any minerals butpumice-ſtone, mountain-leather, and ſome varieties of the opal, which do not equal ourſerpentine in denſity. The chemical experiments we have hitherto made, prove that it con-tains, like the jade or lapis ollaris, oxided iron; but not iron capable of attraction by the mag-net. The ſolutions in muriatic acid, mixed with the nitric acid, are yellow, and not green likethoſe 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 oxida-tion, and that of the iron which colours the ſerpentine, various calcareous ſtones, andperhaps even certain vegetable matters! What difference between a ſubſtance which actsalike on the two extremities of the needle, and a ſtone of which the ſmalleſt portions exer-ciſe a ſpontaneous polarity! Let us purſue the path of obſervation; let us collect indubi-table facts. By this method the theories of natural philoſophy will be eſtabliſhed on ſolidand durable foundations.