Magnetic Characters of Metals, ^c. 85 



gi^eater quantity of ferruginous matter (if present) is brought 

 into operation than in the small pieces. 



40. Cavallo also states, that hammering an unmagnetic 

 piece of brass will cause it to become magnetic. An instance 

 of this kind I have never yet met with ; but I have found that 

 when an unhammered piece has been so slightly magnetic as 

 to have that character but just discernible, hammering it so 

 as to compress its two sides closer together, gives it an in- 

 creased magnetic action, which may possibly be a conse- 

 quence of bringing the whole of its magnetic particles more 

 completely within the range of the testing magnetic influence ; 

 and I am inclined to believe that, had Cavallo' s test been 

 more powerful than a magnetic needle, he would have found 

 that those pieces whose magnetism he thought was due to 

 compression alone were slightly magnetic previously. Still, 

 however, there is a possibility that magnetism might be de- 

 tected in compressed brass, in which that powder is too feeble 

 to be detected whilst the metal is in an uncompressed state, 

 even by powerful magnetic tests. 



41. In addition to the advice given by Cavallo, respecting 

 the necessary caution in employing brass in the construction 

 of compass boxes (note to 6), I should advise the makers of 

 those useful instruments to test every piece of brass intended 

 to be employed in their construction, by a powerful magnet, 

 instead of testing them in the usual way by means of a deli- 

 cate magnetic needle ; and if this test be accurately per- 

 formed when the metal first arrives from the foundry, the de- 

 tection of any concealed magnetism, even if very feeble, will 

 be almost certain, and much of that labour and uncertainty, 

 which must always attend examinations by the needle, would 

 be avoided. Unfortunately, however, too much reliance is 

 usually placed on the mere appearance of the brass, or on the 

 character of the foundi'y whence it is procured, and the con- 

 sequence is, that but very few brass compass boxes that are 

 in common use are entirely free from magnetic action. 



42. The next alloy of importance that I have examined ia 

 German silver, in which nickel is one of the principal consti- 



