518 ■ Prof. De la Rive on the Action of Magnetism upon 



iron ball may be observed to be depressed or elevated at the 

 moment of magnetization : this movement evidently depends on 

 a change of direction on the part of the electric fibres which 

 radiate from the ball. 



But the best way of studying the action of magnetism in the 

 case of a magnetized bar placed inside the gas, is to use a bell- 

 glass or cylindrical jar, 16 centims. in diameter by 20 centims. 

 in height, in the axis of which is placed a soft-iron rod about 3 

 centims. in diameter, terminated by a rounded end situated at 

 the middle of the axis of the cylinder. This rod is fixed into a 

 round disk, which serves to close the jar. A metallic ring, about 

 12 centims. in diameter, made of wire about 3 or. 4 millims. in 

 thickness., is situated in a plane perpendicular to the axis of the 

 jar, in such a position that the top of the soft-iron rod is at its 

 centre; this. ring is soldered to a stem covered with an insula- 

 ting coating, through which it is put in communication with one 

 of the poles of a RuhmkorfFs apparatus, the other pole of which 

 is connected outside the jar with the end of the soft-iron rod. The 

 whole of that portion of the rod which extends into the jar being 

 covered with an insulating coating, except the extremity, the 

 discharge passes between this extremity and the ring of which it 

 is the centre. In order to magnetize the soft-iron rod, it is only 

 necessary to put it in contact, by its outside end, with one pole 

 of the electromagnet, a thin sheet of india-rubber being put be- 

 tween, so that the whole apparatus may be well insulated. 



The cylindrical jar is closed at the other end also, where there 

 is no iron rod, and is furnished with two stopcocks, one of which 

 serves for introducing and rarefying the gas, while the other, 

 being constructed in the way devised by Gay-Lussac, allows of 

 the introduction into the jar of a greater or less quantity of any 

 vapour. 



I made a very large number of experiments with this jar, fill- 

 ing it successively with air, nitrogen, and hydrogen at various 

 degrees of rarefaction, and employing these gases sometimes 

 perfectly dry, and sometimes mixed with larger or smaller pro- 

 portions of water- or alcohol-vapour. 



Dry atmospheric air and nitrogen give almost identical results, 

 the only difference being that the light is brighter and more 

 distinct with nitrogen. If the soft iron is made the positive 

 electrode and the ring the negative electrode, at a certain degree 

 of rarefaction the luminous discharge is seen to form a sort of 

 peach-red envelope round the top of the soft iron, and a pale 

 violet sheath extends over a greater or less number of degrees 

 of the. ring. At a very low pressure this sheath surrounds the 

 whole ring, while the top of the soft-iron rod is enveloped by a 

 rose-coloured halo, and a very short jet of the same colour, 



