On the Magnetic Susceptibility of Liquid Oxygen. 327 



the displacement. If, however, an attempt is made to determine the 

 force acting upon a paramagnetic body in a non -uniform field by a 

 balance, the body weighed is in unstable equilibrium. 



We have only recently overcome these difficulties. The method 

 we have adopted for cooling the body under test, is to suspend it 

 freely near the bottom of a test-tube, which is placed in a vacuum 

 vessel, the interspace between the two being filled with liquid air. 

 In this way the body is cooled by radiation to the temperature of 

 liquid air, and yet it is suspended in, and surrounded by, gaseous air, 

 the magnetic susceptibility of which is exceedingly small compared 

 with that of liquid oxygen or liquid air. 



By limiting the vibration of the balance within small limits by 

 the stops, or by gradually varying the field of the magnet with a 

 carbon rheostat, until the field is just able to move the object from a 

 standard position against the fixed restraining force supplied by a 

 constant counter-balancing weight, we have been able to effect the 

 measurements of the apparent weight of the tested object at a given 

 distance from the pole, and in a known field, even though the 

 equilibrium is not stable. In this way we have made preliminary 

 experiments on the variation in the diamagnetic susceptibility of 

 bismuth, and of the paramagnetic susceptibility of manganous sulphate 

 in the solid condition. 



We made a preliminary experiment by weighing in and out of 

 the magnetic field a small closed glass bulb, exhausted of its air 

 both when in ordinary air at the normal pressure and temperature, 

 and then suspended on the dense gaseous air in the inner test-tube, 

 which is at a temperature of 182 C. lying at the bottom of the 

 inner test-tube, placed as above described in a vacuum vessel. We 

 found the magnetic susceptibility of the dense air at 182 C. to 

 be -fO'2Sx!0~ 6 , in other words about 10 times the susceptibility 

 of air at the normal temperature and pressure. This number O28 

 is quite insignificant compared with numbers of the order of 100 or 

 300. Hence an object suspended in the above described manner, can 

 be reduced to the temperature of liquid air without changing the 

 susceptibility of the surrounding medium by an amount which is at 

 all comparable either with that of liquid oxygen, or with the value 

 of the susceptibility of bismuth, or of most paramagnetic bodies 

 -such as the salts of iron, nickel, cobalt, manganese, or of palladium, 

 or any of the strongly paramagnetic bodies. 



In this manner we have made a determination of the change in 

 paramagnetic susceptibility of crystallised manganous sulphate in 

 the form of powder. 



We find that the susceptibility of the salt at 25 C. is to that 

 at 182 C. in the ratio of 105 to 349 or 1 to 3'32. 



These centigrade temperatures, 25 C. and 182 C , correspond to 



VOL. LX1U. 2 B 



