138 ELECTRICITY. 



feet, Volta engaged in the investigation of the electric state of the air. He 

 substituted for the suspended balls two blades of dry straw, hanging in contact 

 and communicating with the lower end of the conducting rod. In addition to 

 this, he had recourse to another apparently strange and unusual expedient. He 

 placed on the point of the rod a taper, so as to cause this conductor to termi- 

 nate in a flame. He contended that the flame attracted to the point of the 

 conductor three or four times as much electricity as would be collected in 

 its absence. This was explained by the effect of the vertical current of air 

 which the flame maintained directly over it, which established a better com- 

 munication between the metallic conductor and the strata of air above it. 



Assuming this property of flame, Volta argued, that since fires robbed the at- 

 mosphere above them of electricity faster and more effectually than metallic 

 points, it must follow that, to prevent coming storms, or to mitigate their force, 

 the best expedient would be to light enormous fires in the middle of extensive 

 plains, or, better still, on elevated stations. If the effects of the lamp on the 

 atmospheric electrometer were admitted, there would be nothing unreasonable 

 in the supposition that large fires may, in a short interval of time, rob immense 

 volumes of air and vapor of their electricity. 



Volta wished to submit this theory to an experiment on a large scale, but 

 Avas not able to carry the design into effect. M. Arago suggested, that by ma- 

 king suitable meteorological observations in those parts of Staffordshire and 

 other English counties which abound in vast iron furnaces, where fires of ex- 

 traordinary magnitude are maintained night and day, and comparing the results 

 with similar observations made in adjoining agricultural districts, the conjec- 

 ture of Volta might be tested.* 



Observations of this kind have accordingly been recently made both in Eng- 

 land arid in certain parts of Italy, the results of which will be explained at the 

 proper place in this volume. 



It has been already stated, that direct observations proved that the atmo- 

 sphere, in its ordinary condition, is always charged with positive electricity. 

 The beginning of the year 1780 was signalized by a capital experiment, by 

 which it was proved that the source whence this vast amount of the electric 

 fluid was derived, or, to speak more correctly, the cause of the disturbance of 

 the general equilibrium of the globe, which gives a surplus of the positive fluid 

 to the air, and leaves the earth surcharged with negative fluid, and which, in its 

 effects, assumes all the terrific forms of the tempest and the hurricane, and 

 piobably of many other violent convulsions which are occasionally exhibited in 

 the war of the elements, is to be found in the process of natural evaporation, 

 which continually maintains its silent and imperceptible progress upon the sur- 

 faces of ocean, lake, and river, and even upon those of organized bodies. That 

 heat passes off in a latent form by such means, and equalizes and moderates 

 the general temperature around us, was well known ; but it was not suspected 

 that the elements of the storm, the coruscations of meteoric light, and the splen- 

 dors of he aurora, were due to the same cause. 



Volta states, that in the year 1778 this idea occurred to him, and that he j 

 conceived the notion of an experiment by which it might be brought to an im- ] 

 mediate trial. Let a metallic dish filled with water be placed on an insulating ) 

 support, and exposed in the open air until it evaporates, the dish being main- < 

 tained in communication with a sufficiently sensible condensing electroscope. | 

 If, in evaporating, the positive fluid be carried off, the dish will, after the evap- 

 ^ oration, be negatively electrical, and the electroscope will show it ; if no!, the 

 ( electroscope will give no sign. Various circumstances prevented Volta from 

 j trying this experiment until the month of March, 1780, when, being in Paris, 



* Eloge de Volta, p. 18. 



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