Intelligence and Miscellaneous Articles. 79 



resistance of the part of the induced current under the influence of 

 the magnet. This resistance is sometimes such that the current 

 may be suddenly interrupted at the instant when the magnet begins 

 to act. This is made evident in the following manner. A tube is 

 taken formed of two parts in communication, one of them present- 

 ing a constriction, the other a different length and diameter. The 

 capillary part is placed in the pole of the electromagnet, after which 

 the current of the coil is started. As long as the magnet is in- 

 active, the light circulates uniformly in the two tubes ; it is sud- 

 denly arrested in the shortest and narrowest at the instant when 

 this is submitted to the action of the magnet. The effect can be 

 produced with chlorine, iodine, sulphur, selenium. 



(2) This cessation of the induced light caused by the magnet can 

 be determined, with the same gas, in two quite distinct cases — 

 either when exhaustion has been carried so far that the induction- 

 current is near the limit which no longer permits it to pass, or, on 

 the contrary, when the tension of the gas is sufficient for the spark 

 to be near the same limit. 



(3) Under the magnetic influence the luminous thread, when it 

 persists, undergoes in capillary tubes a narrowing which can some- 

 times be perceived by simple inspection. This narrowing is pro- 

 duced by an augmentation of resistance, sufficiently energetic at 

 times to be accompanied by a change of tint in the tube, or even by 

 a modification of the spectrum. In certain gases, such as hydrogen, 

 nitrogen, carbonic acid, the influence of the magnet is hardly per- 

 ceptible, and the modifications observed enter into the system of 

 the primitive lines. 



(4) This narrowing, or the change of tint of the luminous thread, 

 does not extend to more than half a centimetre from the poles : 

 thus on taking a tube of sufficient length, by changing the height 

 of the spectroscope while the magnetization is going on, the normal 

 spectrum (that produced by the light outside of the magnetic field) 

 and the spectrum modified by the vicinity of the magnet can be 

 successively seen. 



(5) In order to form a good judgment of the action of the mag- 

 net, it is necessary to manage so that the spectrum is not very 

 bright at starting. As soon as the current passes in the electro- 

 magnet, the lines appear in all their splendour. The phenomenon 

 is particularly successful and gives the most perspicuous results 

 with chlorine, bromine, chloride of tin, fluoride of silicium, and 

 sulphuric acid. 



(6) Direct measurements have proved that, for these last sub- 

 stances, the new lines developed under these circumstances are 

 distinct from those which characterize the normal spectrum of the 

 same gas traversed by a sufficiently energetic induced current out- 

 side the range of a magnet. — Comptes Rendus deVAcad. des Sciences, 

 vol. lxxx. pp. 1161-1164. 



ON THE VELOCITY OF MAGNETIZATION AND DEMAGNETIZATION 

 OF IRON, CAST IRON, AND STEEL. BY M. DEPREZ. 



In pursuing my researches on electromagnets and their applica- 

 tion to the registration of very rapid phenomena, the first results 

 of which have been already communicated to the Academy, I have 



