Chemical Science, 203- 



experiment is considered as bearing upon the theory of the globe, 

 or at least upon that theory which supposes a central fire, since, 

 by contact with the cold, damp superficial parts, the fire may be the 

 active agent in producing magnetic currents. — Bib. Univ., 1828. 



7. Purity of Metals tested by the Galvanometer. — Mr. (Ersted 

 proposes the galvanometer as an indicator of the comparative 

 oxidability of metals, and ako of their comparative purity. The 

 metals in contact with the ends of the galvanometer wire are to 

 be brought together with a portion of some proper fluid between 

 them, and the relation of the two used, one to the other, will be indi- 

 cated of course by the way in which the needle moves, and the 

 extent of the motion. The needle is to be governed not by the 

 magnetism of the earth merely, but by two magnets placed at a 

 proper distance. 



The quantity of copper in an alloy of that metal with silver, may 

 in this way be instantly ascertained. A series of metallic plates 

 are to be prepared, from the purest silver down to mere copper, 

 and of known composition ; then the piece of silver to be tried is 

 to be applied to one or another of these, and both to the galvano- 

 meter, until that plate is found which has the same electro-motive 

 power with the piece of metal : they will then both have the same 

 composition. Muriatic acid slightly diluted is the fluid to be used : 

 it may be placed between the two metals by a piece of linen or 

 well-washed amadou. The surfaces of the two pieces of metal 

 for contact with the acid should be very clean, and exactly of the 

 same size. — Jahrb. der Chemie, 1828. 



8. Construction of Magnetic Needles. — According to M. de Legey, 

 steel for magnetic needles should not be selected from amongst 

 springs, for such steel is formed of fibres more or less hard, which, 

 by the action of the hammer, has had different directions and une- 

 qual hardness given to them. M. Legey prefers German laminated 

 steel plate, from which he cuts a strip in the direction of the length, 

 and then draws it out, so as to close the pores, till it is very brittle. 

 From this plate he cuts the lozenge intended for the needle. All 

 the operations should tend to lengthen the fibres in parallel direc- 

 tions. The steel is then to be hardened, after which it is to be 

 moderately tempered, then polished on the wheel, and finally magr 

 netized to saturation. 



Before magnetizing the needle, it is examined, and usually found 

 to have two poles. Whatever may have caused them, M. Legey 

 regards the needle as more apt to receive magnetism, according to 

 the position of these poles, than in any other direction, and there- 

 fore endeavours to preserve them in every operation to which the 

 needle is subjected; thus, in the polishing it should always be 

 done in the direction of the length of the needles, and the southern 

 pole should be held opposite to the course of the wheel ; a pro- 



