XII. SPECIAL COLLECTIONS. 387 



A sphere of brass is fixed on a bar of india-rubber or ebonite 0-6 meter 

 long, which is screwed to the board of the cross-shaped stand. A cylinder of 

 india-rubber, 3 meters long, is fixed to the same board at about 0-7 meter 

 from the sphere. From the cylinder proceeds a branch with an arm, both of 

 india-rubber. On the arm are fixed 16 Geissler's tubes, the air in which has 

 n pressure of about 0*5 millimeter. The lower ends of the tabes are pierced 

 by platinum wires, which are directed towards the sphere, whilst at the upper 

 end the platinum wires are, by means of their copper wires, in a metallic union 

 with a button, and also in metallic union with the earth. From underneath 

 the sphere a copper wire, well insulated with india-rubber, leads to the negative 

 pole of a Holtz's electric machine (a machine by Carre of Paris was employed 

 with great advantage), of which the positive pole is in metallic connexion 

 with the earth. As soon as the machine is put in movement, the sphere, 

 being charged, becomes negatively electric, and at the same time there goes 

 through all the tubes a current of reddish-lilac light, so that they altogether 

 form a shining bow-shaped belt. With an ordinary machine this pheno- 

 menon may still be observed when the lower ends of the tubes are at a 

 distance of tivo meters from the sphere. This proves evidently that the 

 electricity flowing out from (or into) the sphere not only traverses the 

 la} r er of air that is between, but goes also with such power through the 

 tubes that the gas therein becomes incandescent from the heat that the electric 

 current produces, as is well known. In. order that the electricity might 

 more easily flow out in the air from the sphere, this latter is furnished 

 with points. These points, as well as the metallic union between the 

 upper end of the tubes and the earth, are of no absolute necessity, for the 

 phenomenon may be produced without them, but in that case the distance 

 between the sphere and the tubes must be considerably reduced. 



The light produced by the apparatus proves clearly that a current of elec- 

 tricity may go through a layer of air of ordinary pressure 760 mm without pro- 

 ducing the phenomenon, but if it meets in its way a space of rarefied air of 

 low pressure (from to 30 ram to 40 ram ) the light immediately arises, the 

 current causing the molecules of gas to become incandescent. 



On the Theory of Polar-Light. 



The knowledge we have acquired of the electric state of the earth proves 

 that it is a conducting body, charged with a small quantity of negative elec- 

 tricity, and surrounded by the atmosphere, in general charged with positive 

 electricity. Though this latter might be produced by an influence from the 

 earth, it is still very probable that it proceeds from the process of evaporation, 

 either directly or by the friction of vapour against particles of air. The 

 atmospheric air possesses a very small conducting power for electricity when 

 dry and at ordinary pressure, but the conducting power increases considerably 

 as soon as the air becomes moist and rarefied. It has been proved by expe- 

 riments that the conducting power is highest at a pressure between 5 mm and 

 10 mm , and amounts to 10,000 times that at a pressure of 760 mm . If the 

 rarefaction of the air is carried further than 5 mm , the conducting power 

 diminishes again, but very slowly. It is known that in proportion to the 

 elevation above the surface of the earth the air becomes more and more 

 rarefied according to the law expressed by the formula given by Laplace, and 

 that consequently, at a certain elevation the earth is surrounded by a layer of 

 air that has a pressure of only 5 mm ; the conducting power for electricity in 

 this layer is sufficiently great to allow of its being regarded as a conductor 

 in comparison to the air in the lower regions, and even in the highest. 

 The negatively electric earth is thus surrounded by a conductor of elec- 

 tricity concentric with it. All the positive electricity that attains the region 

 of rarefied air at about 5 mra , or, as it might be called, this air conductor, 



Bb 2 



