Faraday to William Whewell   10 October 1850

Folk[e]stone | 10, Octr. 1850

My dear Sir

I snatch a few moments to send you some account of the points I referred to in my last¹[.] I will not occupy your time by leading you through the successive investigations which are described in two or three papers now sent into the R.S.² but go at once to the chief results. Oxygen is a magnetic body in its gaseous state & rather strongly so. The kind of proof is this. N & S are the poles of an electro magnet & cc are piece of soft iron connecting them but turned away in the middle[.] The consequence is that I have a strong field of action all round the attenuated part equal in force & condition[.] If I take two equal diamagnetic bodies as for instance 2 glass cylinders & suspend these about 1 1/2 or 2 inches apart on a cross bar fixed to the end of a horizontal line supported by a fibre of cocoon silk & then adjust this so that the cylinders hang on opposite sides of the keeper described & then put on the magnetic power immediately the whole arrangement takes up a position in which the glass cylinders are equidistant from the core. If for one of the glass cylinders I substitute a cylinder of bismuth or phosphorous or a tube of water the two stand at different distances & by putting on a force of torsion (by a wire not a cocoon thread) I can measure the relative diamagnetic force of the bodies. I have a differential balance with the capability of measuring very accurately & yet of removing or at least balancing & estimating [the] interfering circumstance.

Now I have the power of examining gases at different pressure & different temperatures a power which I have long been searching after. First I prepare flint glass bulbs obtain them so as to be nearly equal in diamagnetic force[.] Then I fill them with gases & seal them up hermetically & I do this when the gas has been by the air pump reduced to half an atmosphere or a third or as nearly as may be to a vacuum, for I reason that if a body (gaseous) were diamagnetic it ought to become less diamagnetic by rarefaction or if magnetic less magnetic by rarefaction[.]

When two bulbs with Oxygen & Nitrogen were put up the oxygen went close up to the angle & the Nitrogen was carried out. With oxygen at 1 atmosphere & 1/2 atmosphere - the 1 atmosphere went up to the core the other out. A bulb with 1/2 atmosphere Ox against another of 1 atmosphere went out but against another of 1/3 of an atmosphere it went inwards. In fine the denser the oxygen the more magnetic it was & when I put a bubble containing 1/3 of a cubic inch of oxygen against an equal bubble containing only an oxygen vacuum, it went inwards with such power as to require force equal to 0.1 of a grain to carry it back to equidistance. With Nitrogen no such thing happens expansion causes no difference, and three bulbs being Nitrogen 1 atmosphere - Nitrogen Vacuum & Oxygen Vacuum. When compared against each other are alike. Nitrogen is neither Magnetic nor diamagnetic but stands at Zero. Oxygen is magnetic & in proportion to its quantity: Another experiment will give you a notion of the degree[.] I took crystals of proto sulphate of iron which is a pretty magnetic salt & dissolved it in water & diluted it until it was bulk for bulk equal to oxygen on the differential balance it then contained sulphate of iron equal to 17 1/2 times the weight of the oxygen which was opposed to it[.]

Now for another point in the Magnetism of Oxygen[.] Like iron it loses its magnetism or magnetic condition by heat & is at common temperatures in the condition of iron about to alter. You have a paper of mine about the diamagnetic condition of flame air & gases³ in which I shew that air heated by a platina helix is rendered more diamagnetic &c &c. Well all the effect there is due to the oxygen by very careful experiments I find that Oxygen rapidly loses its magnetic character by heat whilst Nitrogen Carbonic acid, &c are unchanged.

All these properties of oxygen it keeps in the atmosphere & I formerly shewed that by cooling the air it was made either more magnetic or less diamagnetic as the case might be. Now I know where the Zero is & that it is the oxygen of the air that was altering in its magnetic relation[.] Again I must refer you to the papers which are long but I trust not too long, for the steps of the reasoning by which I found upon this property of oxygen a branch of physics which I have called atmospheric Magnetism. I think I see the cause in the heating power of the Sun in the atmosphere for all the periodic solar variations of the needle both annual & diurnal & as far as I have gone the results of observations tally with the theory - besides these variations there are many others due to atmospheric changes & I am in hopes we shall be able to refer so many to this cause as to give us a far clearer view of the power as it exists in the Earth as its origin than we have hitherto had & so help us to discriminate that which belongs essentially to its source[.]

The postman is at the door & I must stop[.]

Ever Your Obliged | M. Faraday

Dr Whewell | &c &c &c