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 quatrieme de M. de Humboldt a M. Pictet, sur la Polarite 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 Prevot 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 meme du cote meridional de la sommite 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.