CERAUNICS. 365 



forming an oxide, and from this a salt, of zinc; in which the free 

 electric fluid of the zinc is probably combined ; while the remaining 

 zinc attracts more of that fluid from contiguous bodies. The hydro- 

 gen of the water, being set free, probably carries with it the electric 

 fluid of the decomposed water ; and is both repelled from the zinc 

 and attracted by the copper ; to which it conveys a charge of the 

 electric fluid. If, then, wires be attached to the two plates, and 

 brought in contact, a galvanic current will pass along them, from 

 the copper to the zinc plate ; forming what is called a simple voltaic 

 circle. If we have a series of cells, all containing an acid solution, 

 the zinc plate in each cell having a metallic connection with the copper 

 plate in the next, they will form a compound circle, or galvanic 

 battery: in which, as in the former case, the wire from the last cop- 

 per plate is called the positive, and that from the zinc end, the nega- 

 tive electrode; the ends of the wires being usually termed poles. 

 If these poles be brought near to each other, provided there be at 

 least 50 pairs of plates in action, they will produce vivid ignition, 

 and other chemical effects : and the current may also be used to 

 charge a Leyden jar, or to produce the other effects of common elec- 

 tricity. 



4. Magnetism, so named from the Greek, ^ayi^s, a loadstone, 

 treats of the peculiar properties first discovered in the loadstone, or 

 native magnet ; with their applications, and the means of observing 

 them. The loadstone, is a complex oxide of iron, with a small 

 proportion of silex and alumina. On presenting it to iron filings, it 

 attracts them ; and two opposite parts of it, which collect them most 

 abundantly, are called the poles. If a piece of soft iron be brought 

 near to a magnet, it becomes magnetized by induction : the part 

 which is nearest to the north pole of the magnet becoming a south 

 pole, and vice versa. As dissimilar poles always attract each other, 

 and similar poles repel each other, the preceding fact explains why 

 the magnet attracts soft iron ; and why it points in a fixed direction 

 in regard to the earth, which really acts as a large magnet would, 

 upon the compass needle. The magnetic poles of the earth do not 

 coincide with its geographic poles ; and hence arises the variation 

 of the compass. As we approach either magnetic pole of the earth, 

 the dissimilar pole of the needle is attracted downwards, and hence 

 results the magnetic dip. The property by which one magnet acts 

 upon another, is called polarity. 



Soft iron, is speedily magnetized by induction ; but loses its mag- 

 netism, as soon as the magnet which excited it is removed. Hard 

 steel, on the contrary, is magnetized slowly ; but retains its magnet- 

 ism for a long time. The best mode of magnetizing a bar, is by 

 placing the opposite poles of two strong magnets at the ends of it; 

 then rubbing it from the centre towards the ends with the poles of 

 another magnet ; each end being rubbed with a pole dissimilar to 

 that which is to be produced ; which is the method of single touch : 

 or else rubbing it from end to end with two dissimilar poles kept 

 near to each other ; the rubbing north pole being nearest to that end 

 which is to become a south pole ; in which consists the method by 

 double touch. Pieces of soft iron, called keepers, or armatures, are 



2H2 



