Faraday to Arthur-Auguste De La Rive   4 February 1851

Royal Institution | 4 February 1851

My dear De la Rive,

My wife & I were exceedingly sorry to hear of your sad loss¹: it brought vividly to our remembrance the time when we were at your house; and you, & others with you, made us so welcome². What can we say to these changes but that they shew by comparison the Vanity of all things under the Sun³. I am very glad that you have spirits to return to work again; for that is a healthy & proper employment of the mind under such circumstances[.]

With respect to my views & experiments⁴ I do not think that anything shorter than the papers (& they will run to above 100 pages in the Transactions⁵) will give you possession of the subject; because a great deal depends upon the comparison of observations in different parts of the world, with the facts obtained by experiment and with the deductions drawn from them: but I will try to give you an idea of the root of the matter. You are aware that I use the phrase line of magnetic force to represent the presence of magnetic force & the direction (of polarity) in which it is exerted; and, by the idea which it conveys one obtains very well, and I believe without error, a notion of the distribution of the forces about a bar magnet or between near flat poles presenting a field of equal force; or in any other case. Now if circumstances be arranged so as to present a field of equal force, which is easily done as I have shewn by the Electro magnet, then if a sphere of iron or nickel be placed in the field it immediately disturbs the direction of the lines of force for they are concentrated within the sphere. They are, however, not merely concentrated but contorted; for the sum of forces in any one section across the field is always equal to the sum of forces in any other section; and therefore their condensation in the iron or nickel cannot occur without this contortion. Moreover the contortion is easily shewn by using a small needle, (1/10 of an inch long) to examine the field: for, as before the introduction of the sphere of Iron or nickel, it would always take up a position parallel to itself: afterwards it varies in position in different places near the sphere. <-> That being understood let us then suppose the sphere to be raised in temperature; at a certain temperature it begins to lose its power of affecting the lines of magnetic force, and ends by retaining scarcely any; so that as regards the little needle mentioned above, it now stands every where parallel to itself within the field of force. This change occurs with iron at a very high temperature & is passed through within the compass, apparently, of a small number of degrees: With Nickel it occurs at much lower temperatures being effected by the heat of boiling oil.

Now take another step. Oxygen as I shewed above three years ago in the Philosophical Magazine for 1847, vol. 31, p.p. 410, 415, 416⁶, is magnetic in relation to Nitrogen & other gases. E. Becquerel, without knowing of my results, has confirmed & extended them in his paper of last year⁷, and given certain excellent measures. In my paper of 1847 I shewed, also, that oxygen (like iron & nickel) lost its magnetic power & its ability of being attracted by the magnet when heated p.417; and I further shewed that the temperatures at which this took place was within the range of common temperature; for the oxygen in the air i.e the air altogether increased in magnetic power when cooled to 0˚F. page 406.

Now I must refer you to the papers themselves for the (to me) strange results of the incompressibility (magnetically speaking) of oxygen and the inexpansibility of nitrogen & other gases:- for the description of a differential balance by which I can compare gas with gas: or the same gas at different degrees of rarefaction:- for the determination of the true zero or point between Magnetic & diamagnetic bodies:- and for certain views of magnetic conduction & polarity. You will there find described certain very delicate experiments upon diamagnetic and very weak magnetic bodies concerning their action on each other in a magnetic field of equal force: the magnetic bodies repel each other and the diamagnetic bodies repel each other; but a magnetic & a diamagnetic body attract each other: and these results combined with the qualities of oxygen as just described convince me that it is able to deflect the lines of magnetic force passing through it just as iron or nickel is; but to an infinitely smaller amount; and that its power of deflecting the lines varies with its temperature & degree of rarefaction.

Then comes in the consideration of the atmosphere and the manner in which it rises & falls in temperature by the presence & absence of the Sun. The place of the great warm region nearly in his neighbourhood;- of the two colder regions which grow up & diminish in the northern & southern hemispheres as the sun travels between the tropics;- the effect of the extra warmth of the Northern hemisphere over the Southern;- the effect of accumulation from the action of preceding months;- the effect of dip & mean declination at each particular station;- the effects that follow from the noncoincidence of magnetic and astronomical conditions of polarity, meridians and so forth;- the results of the distribution of land & water for any given place;- all these and many other things I must refer you for, to the papers. I could not do them justice in any account that a letter could contain, and should run the risk of leading you into error regarding them. But I may say that, deducing from the experiments & the theory what are the deviations of the magnetic needle at any given station, which may be expected as the mean result of the heating & cooling of the atmosphere for a given Season & hour, I find such a general accordance with the results of observations, especially in the direction & generally in the amount for different seasons of the declination variation, as to give me the strongest hopes that I have assigned the true physical cause of those variations and shewn the modus operandi of their production.

And now, my dear De la Rive I must leave you and run to other matters. As soon as I can send you a copy of the papers I will do so and can only say I hope that they will meet with your approbation. With the kindest remembrances to your Son⁸ Believe me to be My dear friend

Ever Truly Yours, | M. Faraday