Mr. J. Tatum on Electro-Magnetism. 107 



" Others have asserted, that the increased temperature 

 arises from the decomposition of pyrites, which abounds in 

 coal and the accompanying strata, and that this is the cause 

 of the high temperature of hot springs. This opinion, how- 

 ever, does not seem to be well founded. Although in the very 

 extensive coal-mines of Great Britain, pyrites abound in great 

 quantities ; yet in no instance was pyrites ever found decom- 

 posed in situ, although the coal abounds with water, and gives 

 out carbonic acid gas and carburetted hydrogen, but never 

 atmospheric air, and the pyrites only decomposes when ex- 

 posed to the action of oxygen. Had pyrites been Uable to 

 decompose in situ, the greater part of the coal-fields in the 

 world would have been destroyed by spontaneous ignition; 

 but this spontaneous ignition only takes place in coal-mines 

 where the pyrites is thrown into the waste, and exposed to 

 the action of atmospheric air, and the moisture of the strata. 

 If pyrites is the cause of the high temperature of hot springs, 

 these springs would vary continually, both in temperature and 

 composition, accorduig to the extent of surface exposed to the 

 decomposing action." ( E.) 



XX. On Electro-Magnetism. By Mr. J. Tatum. 



To the Editors of the Philosophical Magazine andjouriial. 



T EMBRACE this opportunity of sendmg you part of a se- 

 -*- cond paper, read before the City Philosophical Society, on 

 Electro-magnetism. 



It will be recollected, that in my former paper I observed 

 that there were eight parts on each side of the equator of the 

 needle which ajipeared to be attracted and repelled by the con- 

 necting wire of the Voltaic apparatus. 



I also closed that paper by showing, that these attractions and 

 repulsions were the reverse on that part of the wire connected 

 with the zinc (or positive) side of the apparatus, to what they 

 were with the part connected with the copper (or negative) side. 



I next wished to ascertain if it were possible to demonstrate 

 an uniform direction of any particular part of the needle 

 round the positive and negative wires. 



Exp. 1. For this purpose I brought the tip of one side of the 

 north end of a dipping needle (on one end of which was at- 

 tached a piece of copper wire to counteract its diji) parallel 

 with, and near the left side of, the negative wire DC, (fig. 13 

 of the last Number of the Phil. Mag.) when it descended. 



Exp. 2. 1 then brought it under the wire, and it turned to the 

 right. 



Exp. 3. It was next plawd near the right side of tlie wire, 

 when it ascended. O 2 Exp. 



