294 Profs. J. A. Fleming and J. Dewar. On the 



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 mag- 

 netic field. Under these conditions a vacuum bulb of very thin 

 glass would behave like a strongly diamagnetic body, and if the mag- 

 netic susceptibility of the vacuum bulb or test mass is & 15 and that of 

 liquid oxygen is & 3 for equal volumes, then the apparent diamagnetic 

 susceptibility of the mass will be (& 2 ki), and the actual para- 

 magnetic susceptibility of liquid oxygen may be deduced fro in a 

 knowledge of &i and (& 2 &i) 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. 1*00287, as determined by us for the magnetic 

 permeability of liquid oxygen, shows that the magnetic susceptibility 

 (&) per unit of volume is 228/10 6 . It is interesting to compare 

 this value with the value obtained by Mr. Town send for an aqueous 

 solution of ferric chloride, and which he states can be calculated by 

 the equation 



10 & = 91-610 0'77, 



where w is the weight of salt in grams per cubic centimetre, and k 

 the magnetic susceptibility. Even in a saturated solution, w cannot 

 exceed O6, hence, from the above equation, we find the value of the 

 magnetic susceptibility of a saturated solution of one of the most para- 

 magnetic iron salts, viz., ferric chloride, is 54/10 6 for magnetic forces 

 between 1 and 9. This agrees fairly well with other determinations 

 of the same constant. On the other hand, the magnetic suscepti- 

 bility of liquid oxygen for the same volume is 228/10 6 , 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, how- 

 ever, 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. B. 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 



* See 'Phil. Trans.,' A, rol. 187, 1896, "Magnetisation of Liquids," J. S. 

 Townsend, M.A. 



