Dr. Draper on the Galvanic Spark, 349 



How far llie experiments given in this memoir bear upon 

 that part of Dr. Faraday's researches in which he has de- 

 termined the relation of common and voltaic electricity by 

 measure, would form a most important subject of investi- 

 gation. The results at which he arrives, are in themselves 

 very astonishing, and are fully borne out by his decisive ex- 

 periments ; but when we come to reflect that these results 

 were obtained by the magnetic 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 indivisible 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 know- 

 ing its condition as to condensation ? Can we know ho'w much 

 electricity is upon a prime conductor, or compare it with that 

 evolved by a voltaic pile, without first knowing its state of 

 condensation? I shall be forgiven for employing these ex- 

 pressions in an unusual way, and for reasoning about this 

 subtile agent as though it were a ponderable body, in as much 

 as it serves, without introducing any hypothesis, to give us 

 more tangible and distinct ideas of what we might otherwise 

 vainly attempt to express. 



In the December Number of this Journal, (L. & 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 ten- 

 sion before contact, it ought to take effect at an unlimited di- 

 stance 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 mercury made to rise gradually 

 towards its point, the spark does not pass until apparent con- 

 tact takes place : this was found in an analogous but vain at- 

 tempt to show the thermo-electric spark. It cannot however 

 be entirely, as that philosopher supposes, " simply a phae- 

 nomenon of combustion," as it is difficult to understand how 

 mercury can enter into combustion in a vacuum*. 



Hampden Sydney College, Virginia, 

 Feb. 22, 1839. 



[* May not the spark, in this instance, result from the combination of 

 mercury and iron, under the influence of electricity, and attended by the 

 evolution of heat and light, and thus still be " simply a phaenomenon of 

 combustion"? E.W.B.] 



