90 ELECTRIC SPARK BEFORE CONTACT IN VACUO. 



332. How far the experiments given in this memoir bear upon that part of Dr. FARA- 

 DAY'S researches, in which he has determined the relation of common and voltaic electricity 

 by measure, would form a most important subject of investigation. The results at which 

 he arrives are in themselves very astonishing, and are fully borne out by his decisive 

 experiments ; but when we come to reflect that these results were obtained by the mag- 

 netic needle and electro-chemical action, we may, perhaps, pause. We may ask whether 

 it is possible to determine by either of these means the absolute quantity of electricity 

 that passes. Both measure, so to speak, the volume that flows, the one in an indivisi- 

 ble portion of time, the other that which has flowed at the end of a finite time ; but do 

 either of them measure the true absolute quantity ? Can we tell the absolute amount 

 of a gas without first knowing its condition as to condensation 1 Can we know how 

 much electricity is upon a prime conductor, or compare it with that evolved by a vol- 

 taic pile, without first knowing its state of condensation ? I shall be excused for em- 

 ploying these expressions in an unusual way, and for reasoning about this subtle agent 

 as though it were a ponderable body, inasmuch as it serves, without introducing any 

 hypothesis, to give us more tangible and distinct ideas of what we might otherwise 

 vainly attempt to express. 



333. In the December number of this journal (L. and E. Phil. Mag., vol xiii., p. 401), 

 which has just reached me, I find some remarks of Dr. JACOBI on the galvanic spark. 

 Some time ago I came, by another method of experimenting, to the same conclusion. 

 If this spark be really projected by the tension before contact, it ought to take effect at 

 an unlimited distance in a perfect vacuum ; but it will be found, on making the trial, 

 that if an iron electrode be sealed into the upper part of a barometer tube, and the mer- 

 cury made to rise gradually towards its point, the spark does not pass until apparent 

 contact takes place : this was found in an analogous, but vain attempt to show the ther- 

 mo-electric spark. It cannot, however, be entirely, as that philosopher supposes, " sim- 

 ply a phenomenon of combustion," as it is difficult to understand how mercury can enter 

 into combustion in a vacuum. 



CHAPTER IX 



ON THE ELECTROMOTIVE POWER OF HEAT. 



{From the London and. Edinburgh Philosophical Magazine for June, 1840.) 



CONTENTS : Object of the Memoir. Experimental Arrangement to determine the Elec- 

 tromotive Power. Temperatures calculated from Quantities of Electricity. Increase 

 of Tension with increase of Temperature. Depends on increased Resistance to Con- 

 duction. Quantity of Electricity independent of heated Surface. In Thermo-electric 

 Piles, the Quantity of Electricity proportional to the Number of Pairs. Best Forms 

 of Construction of Thermo-electric Pairs. 



334. FROM the memoir of M. MELLONI, on the Polarization of Heat, inserted in the 

 second part of the first volume of the Scientific Memoirs, we learn that M. BECQUEREL, 



