152 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



THE MAGNETIZATION OF GLASS. 



An interesting paper in Poggendorf's An?ialen, by Pro- 

 fessor Villari, treats of the time that flint-glass takes to be 

 magnetized, demagnetized, and to turn the plane of polariza- 

 tion. He rotated a glass cylinder between the poles of an 

 electro-magnet, where it acted like a cylindrical lens with 

 polarized light passing through the poles. When not mag- 

 netized, the cylinder, whether in motion or at rest, was neu- 

 tral to the light ; but when magnetized, its plane rotating 

 power diminished considerably with increasing velocity of 

 rotation ; the reason being that, in such quick revolution, 

 each diameter remained too short a time in the axial direction 

 to receive all the magnetism it would otherwise have acquired. 

 To give flint-glass such diamagnetic intensity as became ob- 

 servable by rotation of the plane required at least one eight- 

 hundredth of a second of time : while to srive it all the diamacr- 

 netism it is capable of taking under a strong magnet, at least 



one four-hundredth of a second was needed. 12 A, 1873, IX., 

 94. 



THE MAGNETISM OF ELECTRO-MAGNETS. 



M. Gaugain, as the conclusion of an extensive investigation 

 into the construction of electro-magnets, states that, in order 

 to explain his observations, it is necessary to conclude that a 

 bar of iron contains two shells as it were of positive magnetism, 

 separated by a layer of negative magnetism. One can easily 

 see how to superpose a greater or less number of layers, al- 

 ternately positive and negative, by passing through the coils 

 of the electro-magnet a moderate number of alternately posi- 

 tive and negative currents of diminishing intensity, the law 

 of diminution not being a matter of chance. It is, however, 

 necessary that the intensity of the induced current should 

 decrease with a definite rapidity. When this intensity di- 

 minishes too slowly, the iron which is submitted to its influ- 

 ence is in the same state as if it had been submitted only to 

 the influence of a current feebler than that which has been 

 employed. It is then feebly magnetized, and, if subjected to 

 the action of a current of greater intensity, it is magnetized 

 equally in both the positive and negative senses. It then 

 behaves as if it were really in a neutral state. Consequently, 



