1839] 



Trap Dykes in the Sienlte of Amhoor. 



301 



of which was hotter than the other. I cannot pass by this envious expe- 

 riment, without attracting attention to the interesting bearing it has 

 upon the phenomena of the clay and mud, previously described under 

 class 6. The circumstances of each are exactly similar, for, it will be 

 remembered, the phenomena I described followed the evaporation of the 

 water mixed with the clay, and it is most natural to suppose, that the 

 sun's heat would not operate on every part with the same degree of in- 

 tensity ; hence then we have the exact arrangement of Nobili's experi- 

 ment viz. moist clny, of variable temp M-ature, in contact throughout. We 

 may therefore conclude, that electric currents wouLI traverse it. Furthe^' 

 on this point, it is known, that evaporation is itself one of our most fruit- 

 ful sources of electricity, and if the water evaporated holds any substance? 

 either ea,rthy or saline, in solution, the quantity of this is proportionally 

 greater; the currents circulating through the clay would thus be increase 

 ed in quantity, though their intensity M^ould b-:" small : but, as wi'l l)e a^'ter- 

 wards proved, this is the state best adapted for producing those effects 

 attributed to them in this paper. 



To prove further that it is a mitter of indiiFerence whether the circuit 

 gains or loses heat, provided only that variation takes pla ;e, I may give a 

 hasty sketch of some observations m nde on thi^ subject* by Oeisted, the 

 celebrated discoverer of electro-magnetism. He made a hexagon, the 

 alternate sides of which were antimony and bismuth, the two metals 

 whose junction gives rise to the strongest currents.: First he heated one of 

 the soldered angles, and immediately the galvanometer needle was de- 

 flected, when two alternate angles being heated the deflection became 

 greater; and greater still, when the spirit lamp was applied to three. 

 Changing his | Ian, he reduced the temperature of one angle by means of 

 ice (doing which was vertically raising the opposite) and, as might be 

 anticipated, the needle was again most sensibly affected. On applying 

 temperature to these alternate angles, and ice to the intermediate ones, 

 Oeisted obtained the greatest effect from the currents— thus shewing 

 that the intensity depended on the ditference between the respective 

 temperatures of the elements of the circuit. Mr. Becqueral arrived at a 

 similar result, finding that the intensity increased with the heat, though 

 a limit seemed ultimately to be put to the law, varying in different sub- 

 stances. To shew that the electricity derived from the above sources is 

 identical both with that from the machine, and the galvanic pile, I 

 may mention, that one Italian philosopher has succeeded in decomposing 



* Dr. Roget's Treatise on Electricity.— Society Useful Knowledge, 



