1821.] On Electro-magnetic Experiments. 137 



remaining in the retort* ; at a lower heat, it gives over a h'ght oil. 

 I have no means of ascertaining its ultimate constituents ; but 

 have little doubt that the above characters are sufficient to 

 authorize our considering it as distinct from petroleum, asphalt, 

 and elastic bitumen, the only three species (with the exception, 

 of retinasphalt, which occurs under very different geognostic 

 relations, and appears to be the result of a very different natural 

 process) into which substances of this class have hitherto been 

 divided by mineralogists. 



The Hatchetine is found filHng small contemporaneous veins 

 lined with calcareous spar and small rock crystals (termed the 

 Mezthyr diamonds) in the ironstone. 



Article XIII. 



Electro-magnetic Experiments. 

 (To the Editor of the Annals of Fhilosophy.) 



SIR, Cambridge, Jan. 22, 1821. 



If, as I imagine, the following electro-magnetic experiments 

 are new, you will oblige me by inserting them in the next number 

 of the Antials of Philosophy. 



A weight was suspended from a small horse-shoe magnet ; on 

 connecting the north pole of the magnet with the copper side of 

 a pair of galvanic plates, the weight was attracted more strongly ; 

 on reversing the wires, it fell. 



A small magnetic bar being placed in the galvanic circuit, its 

 south pole bemg in connexion with the positive end of the bat- 

 tery, the magnetism was destroyed in half a minute. 



A connecting wire, l-15th of an inch diameter, being placed 

 horizontally in the plane of the magnetic meridian, over a com- 

 pass, the deviation of the needle was between 80° and 90°; in 

 Oersted's experiments, it is stated to be about 45°. 



On using a smaller wire, the deviation was diminished ; and 

 when the diameter of the wire was l-200th inch, it was less 

 than 20°. 



This diminution of the deviation took place whether the small 

 wire were immediately over the compass, or interposed in any 

 other part of the circuit. 



When the connexion was made at the same time by the two 

 above-mentioned wires, the smaller being three inches long, the 

 longer five feet, the needle of a compass placed under the smaller 

 deviated about 10° ; that of another under the larger deviated 



* Elastic bitumen, at the same heat, gives over a yellowish oil perfectly fluid. 



