Page:Proceedings of the Royal Society of London Vol 60.djvu/320

294 oxygen with, far greater accuracy. This method consists in observing the mechanical force which acts upon a vacuum bulb or mass of matter of known and very low susceptibility when it is suspended free from gravity in a vessel of liquid oxygen, and in a variable magnetic field. Under these conditions a vacuum bulb of very thin glass would behave like a strongly diamagnetic body, and if the magnetic susceptibility of the vacuum bulb or test mass is Teu and that of liquid oxygen is Jc2 for equal volumes, then the apparent diamagnetic susceptibility of the mass will be — (&* — x), and the actual paramagnetic susceptibility of liquid oxygen may be deduced from a knowledge of hi and — (h2 — hx). By this method we hope to be able to determine whether the permeability of liquid oxygen is a function of the magnetising force. The latest experimental results and measurements made with solutions of iron salts, such as those made recently by Mr. J. S. Townsend,* appear to show that the magnetic permeability of solutions of these iron salts is a constant quantity at least* for a range of magnetic forces varying from 1 to 9 C.G.S. units.

The value, viz. l -00287, as determined by us for the magnetic permeability of liquid oxygen, shows that the magnetic susceptibility (h) per unit of volume is 228/10®. It is interesting to compare this value with the value obtained by Mr. Townsend for an aqueous solution of ferric chloride, and which he states can be calculated by the equation

10® fc = 91‘6 w — 0'77,

where w is the weight of salt in grams per cubic centimetre, and h the magnetic susceptibility. Even in a saturated solution, w cannot exceed 0‘6, hence, from the above equation, we find the value of the magnetic susceptibility of a saturated solution of one of the most paramagnetic iron salts, viz., ferric chloride, is 54/10® for magnetic forces between 1 and 9. This agrees fairly well with other determinations of the same constant. On the other hand, the magnetic susceptibility of liquid oxygen for the same volume is 228/10®, or more than four times as great. The unique position of liquid oxygen in respect of its magnetic susceptibility is thus strikingly shown. It is, however, interesting to note that its permeability lies far below that of certain solid iron alloys generally called non-magnetic.

The 12 per cent, manganese steel of Mr. R. A. Hadfield is usually spoken of as non-magnetic, yet the magnetic permeability of this last substance has been shown to be 1*3 or 1*4.

We have applied the foregoing method also to the determination of the magnetic permeability of liquid air. Since liquid air which

Townsend, M.A.
 * See ‘ Phil. Trans./ A, vol. 187, 1896, “ Magnetisation of Liquids,” J. S.