1839] 



Trap Dt/les in the Sienite of Amhoor. 



303 



of the needle, and the obtaining of sparks, are the chief links of that 

 chain which binds them to one common source. The last to which this 

 has been extended is the animal electricity, or that derived from those 

 animals, as the torpedo, to which God has given the power of adminis- 

 tering shocks, as their weapons of defence. Professor Linari of Sienna 

 has obtained both the direct and the induced spark from the torpedo, 

 and Dr. Davy has decomposed water by its means. " It has been far- 

 ther proved, that the apparent differences are due to the peculiar state 

 in which the electricity respectively exists, with relation both to its 

 quantity and its intensity." Now, according to Dr. Roget, these states 

 are three in number, first the state of highest tension, or that in which 

 electricity derived from the common machine exists, when it accumulates 

 till its force is sufficient to enable it to make its way through the air, 

 which is nearly a perfect non-conductor. Second, in a similar degree 

 of tension as when derived from the galvanic battery, where the path is 

 more open to it, and therefore it does not accumulate so much as in the 

 common battery. The metallic parts of the galvanic battery are good 

 conductors, but the fluid ones fire not so easily traversed, the quantity 

 circulating is therefore greater, though its intensity is smaller than in 

 the preceding case. Third, in the smallest of all degrees of tension, as 

 in thermo-electric currents, throug^iout the whole of which no impedi- 

 ment to the free passage of the electricity is met with, the circulation 

 taking place, as it were, from particle to particle, without any non-con- 

 ducting medium being interposed as in the two first cases. The 

 peculiarity therefore of thermo-electric currents is, that while their in- 

 tensity is next to nothing, the quantity is comparatively very great. Now . 

 it will be considered as a striking fact in connection with this enquiry, 

 when it is known, that batteries whose principles of action are the clos- 

 est possible to those of thermo-electric circuits, have been extensively 

 used as the means of producing crystillization in a very great number of 

 bodies. Those singular experiments due to Mr. Crosse, and which, from 

 the sensation they excited, are still doubtless fresh in the memories of all, 

 were performed under the circumstances above referred to. Feeble cur- 

 rents, derived from batteries excited by no stronger agent than pure 

 w^ater, bat continued for a long time, were the means in operation by 

 which crystals of quartz, arragonite, carbonate of lead, lime and copper, 

 &c. were produced. These experiments I partially repeated some time 

 after their announcement with most satisfactory results. I emploj^ed 

 more powerful batteries, and obtained crystals more rapidly, but these are 

 never so perfect as when formed by long continued action. Mr. Becque^ 

 ral, who has with much success pursued this new path, found that j'earfe 



