Magnetic Characters of Metals^ ^c. 71 



tallic bodies, are displayed by alloys of iron with other me- 

 tals.* 



6. Tiberius Cavallo was amongst the earliest inquirers 

 into the magnetic action of non-ferruginous metals ; but the 

 principal part of his experiments were limited to copper and 

 brass, specimens of both of which he found to be magnetic ; 

 and especially after they had suffered the operation of ham- 

 mering. The investigating apparatus of this philosopher, 

 like that of many subsequent inquirers, consisted of a delicate 

 magnetic needle, to the poles of which the specimens under 

 examination, to prevent commotion in the air, were slowly 

 and dexterously presented -t 



* At the time the first part of this memoir was read, I was not aware of these 

 curious and interesting facts. They were subsequently discovered, and are de- 

 scribed in the second part. 



t The following are the conclusions at which Cavallo arrived respecting the 

 magnetism of brass : — 



'' 1st, Most brass becomes magnetic by hammering, and loses its magnetism 

 by annealing or softening in the fire, or, at least, its magnetism is so far weak- 

 ened by it, as afterwards to be only discoverable when set to float in quick- 

 silver. 



" 2d, The acquired magnetism is not owing to particles of iron or steel im- 

 parted to the brass by the tools employed, or naturally mixed with the brass. 



" 2d, Those pieces of brass which have that property, retain it without any 

 diminution after a great number of repeated trials, viz., after having been re- 

 peatedly hardened and softened. 



*• ^th, A large piece of brass has generally a magnetic power somewhat 

 stronger than a smaller piece, and the flat surface of the piece draws the needle 

 more forcibly than the edge or corners of it. 



•* 5th, If only one end of a large piece of brass be hammered, then that end 

 alone will disturb the magnetic needle, and not the rest. 



" Qth, The magnetic power which brass acquires by hammering has a certain 

 limit, beyond which it cannot be increased by further hammering. This limit 

 is various in pieces of brass of different thicknesses, and likewise of different 

 qualities. 



" 1th, Though there are some pieces of brass which have not the power of 

 being rendered magnetic by hammering, yet all the pieces of magnetic brass 

 that I have tried lose their magnetism, so as no longer to affect the magnetic 

 needle, by being made red hot, excepting, indeed, when some pieces of iron are 

 concealed in them, which sometimes occurs ; but in this case the piece of brass, 

 after having been made red hot and cooled, will attract the needle more forci- 

 bly with one part of its surface than with the rest of it ; and hence, by turning 

 the piece of brass about, and presenting every part of it successively to the 



