007 



of an electromagnet f upon the piece of iron A'; tlie electro-magnet 

 is moved from outside tlie apparatus by means of two bronze strips 

 L, vvliicl) I'un over tlie puUies N and pass througli the walls without 

 friction, terminating in two elastic enclosures similar to F and ?j. 

 These suspension-strips, which also serve as conductors for the 

 electro-magnet, are moved by hand. 



The carrier is provided at B with a small stopper serving as a 

 safety valve, to guard against a casual excess of pressure in the 

 carrier during the measurements, when the tap is closed, bursting 

 the carrier. 



The preparations for a series of measurements took place in I he 

 following oi-der : when the enclosure is found to be air-tight, a 

 sufficient weight is placed upon the plate Q to bring the carrier 

 to its lowest position; the connection with the tube E can then be 

 made, the tap C is then opened, and any gases that may be con- 

 tained in the carrier, are pumped out. Then the cryogen bath (of 

 pure liquid nitrogen) is made ready, and the temperature reduced 

 to a few degrees below the boiling-point by reduction of the pressure. 

 Finally the desired amount of pure oxygen, which has been previously 

 measured in a volumenometer resembling a TöPL?:R-pump, can be 

 condensed in the carrier, and then by means of the same instrument, 

 the quantity of pure nitrogen required to completely till the experi- 

 mental tube R. After this C is shut, D is disconnected, the gas 

 supply tube is pulled back and the overweight taken away ; the 

 carrier is then free, and completely closed against the vapour of the 

 bath. While the bath is being brought to atmospheric pressure, and 

 to an even temperature, which is greatlj^ promoted by the pump-stirrer 

 P '^), which causes a vertical circulation in the whole bath, the 

 electro-magnet which serves as stirrer for the mixture, is put in 

 motion, until the magnetic attraction, which is measured from time 

 to time, reaches a constant value. 



The question of what the composition of the mixture is deserves 

 particular attention. A given weight of oxygen is introduced into 

 the tube, which spreads over a given volume, known by the cali- 

 bration of the experimental tube. The weight of oxygen per cm'., 

 i. e. the concentration, is thus given by immediate experiment, 

 which quantity is also the most important from the magnetic point 

 of view, as the measurements in the first place give the suscepti- 

 bility of the mixture, and the concentration enables us to deduce 

 from it the specific magnetism which can be ascribed to the oxygen 



1) See H. Kamerlingh Onnes, Comm. No. Ii23, § 2. 



