CASE OF THERMO-ELECTRICITY AND MACHINE ELECTRICITY. 



89 



rectly as those numbers. This is abundantly shown by the experiments of LANE, HAR- 

 RIS, and other philosophers. 



328. Now upon what does this striking distance depend ! Plainly upon the elastic 

 force of the coerced fluid, and therefore the striking distance will measure the elastic 

 force or tension. We condense upon a given surface increasing quantities of the elec- 

 tric fluid, and find that the law in relation to its elastic force is, that the tension of a 

 given quantity is inversely as its volume. But this is the law of MARRIOTTE in relation 

 to the ponderable elastic fluids. 



329. The following numerical determinations were made by adding successive plates 

 to the first single hydro pair, and taking the values of the current on each addition. It 

 is offered merely as an illustration of the chief fact under discussion, and is not to be 

 regarded as absolutely correct, though every precaution was taken to avoid changes in 

 the current. It shows the decrease of quantity and the increase of tension in VOLTA'S 

 instrument. Of course, in reasoning upon it, the hypothetical action of each plate is 

 assumed to be equal to that of any other in the series. 



TABLE M. 



330. Thermo-electric piles are well known to give the same general results, as respects 

 tension, that hydro-electric piles do ; they are much better suited to the purpose of the 

 experimenter, and give currents that are far more constant. The following table rep- 

 resents the action of such a battery, consisting of wires of copper and tinned iron, each 

 element being about one foot long and T ' T inch in diameter. The source of heat was 

 a bath of boiling water. 



TABLE N. 



331. The beautiful experiments of BECQUEREL, and the equally elegant repetition of 

 them by Dr. GOLDING BIRD, show that the view I have here taken of the action of a sin- 

 gle pair is correct. The latter chemist found, that not only could a single pair decom- 

 pose bodies, such as iodide of potassium, &c., which easily yield up their elements, but 

 that the ammoniacal amalgam might be formed, potassium reduced, and, in point of fact, 

 any decomposition effected. And what is the plan followed ! The current is forced to 

 pass, in the electrolyte that is to be decomposed, an obstacle or resisting medium ; the ten- 

 sion instantly rises, but at a vast sacrifice of quantity, so that the magnetic needle, which 

 measures only the quantity passing in an indivisible portion of time, is barely affected. 

 Yet, by continuing the current for a great length of time, the resulting decomposing 

 effects are finally the same as those obtained more speedily by the action of many pairs. 



M 



