VOL. LXXVI.] PHILOSOPHICAL TRANSACTIONS. 59 



repeatedly hardened and softened. But I have not found any means to give that 

 property to such brass as had it not naturally. 4thly, 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 corner of it. 

 5thly, 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. 6thly, 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 thickness, and also of different quality. 7thly, Though there 

 are some pieces of brass which have not the property of being rendered magnetic 

 by hammering ; yet all the pieces of magnetic brass, that I have tried, lose their 

 magnetism by being made red-hot, excepting indeed when some piece of iron is 

 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 forcibly 

 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 suspended 

 magnetic needle, we may easily discover in what part of it the iron is lodged. 



From those observations it follows, that when brass is to be used for the con- 

 struction of instruments in which a magnetic needle is concerned, as dipping 

 needles, variation compasses, &c. the brass should be either left quite soft, or it 

 should be chosen of such a sort as will not be made magnetic by hammering, 

 which sort however does not occur very easily. 



Examination of the Magnetic Properties of some other Metallic Substances.—-' 

 The result of the experiments on brass induced Mr. C. to examine other metallic 

 substances, and especially its components, viz. copper and zinc : though the re- 

 sult of the experiments has not been very remarkable, excepting with platina, 

 which metal has properties in a great measure analogous to those of brass. — 

 Having examined various pieces of copper, by means of the suspended magnetic 

 needle, and having never found them magnetical, except only sometimes in such 

 places as had been filed, and where some particles of steel might have been left by 

 the file, he next proceeded to hammer some pieces of it, not only in the usual 

 way, but also between flints : the result however was very dubious ; for though 

 generally they had no effect whatever on the needle, yet sometimes he thought 

 the needle was really attracted by some pieces of hammered copper ; but then this 

 attractive power was so exceedingly small as not to be depended on. Zinc, either 

 not hammered, or hammered as far as could be done without breaking it, showed 

 no signs of magnetism whatever, when presented to the magnetic needle. Neither 

 had a mixture of zinc and tin any action on the needle. A piece of a broken re- 

 flector of a telescope, which consisted of tin and copper; a mixture of tin, zinc, 

 and a little copper ; a piece of silver, both soft and hammered ; a piece of pure 



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