On Crystallography, 36S 



electricity. These are exceptions as it were to the ordinary 

 results, susceptible even on that very account of accurately 

 designating: the minerals which present them. 



15. Magnetism. We know that two magnetic needle?, 

 when they turn towards each other their north or their 

 south poles, are repelled ; whereas there is an attraction, if 

 the poles facing each other are one of them south and the 

 other north. In consequence we recognize a needle, i,e, 

 a piece of iron in the state of permanent magnetism, from 

 the same side of this piece of iron presented successively 

 to the two poles of a magnetic bar suspended freely, at- 

 tracting the one and repelling the other, or vice versa. 



But if we employ a piece of common iron, there will 

 be aiiractioniu both cases; the pole nearest the iron will 

 communicate to the part turned towards it a magnetism 

 contrary to its own, so that there will then be two mag- 

 netic needles which will face each other by opposite poles. 

 The magnetism thus acquired is merely instantaneous : it 

 gives place to the contrary magnetism the moment the 

 iron passes from the neighbourhood of one pole to that of 

 the other, and is dissipated tjie instant the iron is no longer 

 in the sphere of activity of the bar. 



7n Tex peri ments of this kind it is preferable to use a 

 needle in the form of a lozenge, three or four inches long, 

 instead of a b^r, the former being more sensible. But the 

 bar would be preferable, if it were requisite for instance to 

 pick out some specimens of iron scattered in ^ piilverulent 

 mass. 



1 shall shdw under the head of oxidulated iron, that most 

 of the crystals, or even the rude masses of this metal, 

 locked up in the bowels of the earth, provided they are not 

 too much oxidized, are two magnets, but of which we 

 cannot observe the polarity except by using a needle slightly 

 magnetic, and this for a reason which I shall give at the 

 ^ame part of my work. 



(Tc be continued.] 



LVII. Account 



