238 Intelligence and Miscellaneous Articles. 



described is precisely equal to the sum of the separate resistances of 

 the portions of the tube of which it is made up ; in other words, 

 the external resistances, of which I am here speaking, may be added 

 to each other like the internal resistances, and consequently they 

 have a constant value, which is independent of the intensity of the 

 electric current. This observation is of some importance, because 

 it proves that the resistance caused by the contact of a glass tube 

 with a metallic ring is not of the same nature as the resistance 

 which is offered by an electrolysed liquid. The latter, as I have 

 shown elsewhere (Comptes Rendust, 24th December, 1855), varies 

 with the intensity of the current, and even with the duration of the 

 electroiysation. I may add that the resistance to the passage of 

 the current which results from electroiysation, never occurs without 

 the electrodes themselves becoming polarized, whereas, in the case 

 of the passage of electricity down a glass tube, I am convinced no 

 polarization takes place. 



It may be said that the resistance which the electric fluid encoun- 

 ters at the contact of the tube with the metallic ring, results 

 from the stratum of air which is interposed between the glass and 

 the metal ; but it must be observed that this stratum of air exists 

 as much in the case of white glass as in that of tube-glass, and that 

 nevertheless the resistance, which with the one of these kinds of 

 glass is very observable, is absolutely imperceptible with the other. 

 It appears to me to be impossible to explain the concurrence of these 

 facts except by means of the new property which I have designated 

 external conducti.bility . 



When the resistance of a conductor is altogether external, as in 

 the case of tube-glass, it would be expected, h priori, to vary with 

 the superficial extent of the metallic rings which connect the tube 

 with the rest of the apparatus ; and this is verified by experience. 

 If, for example, the communications are made by means of broad 

 bands, the resistance is much less than if wires of small diameter 

 are employed. When, on the other hand, the resistance is altoge- 

 ther internal, as in the case of gum-lac, the electric fluid trans- 

 mitted by a cylinder of given length is altogether independent of 

 its fittings, and is the same with fine wires as with broad bands. 



The external resistance of a tube depends on the state of its sur- 

 face, and may be singularly modified by the slightest fi-iction. If 

 one extremity of a long tube, the middle of which has been rubbed 

 with a woollen cloth, be held in the hand, and the other be brought 

 into contact with a previously charged electroscope, the electricity 

 passes away from the latter almost instantaneously, as if the tube 

 .had not been rubbed, thus showing that the internal conductibility 

 of the tube has not been perceptibly modified by the friction it has 

 undergone; but if the electroscope be again charged, and put in 

 contact with that part of the tube which has been previously rubbed, 

 the electricity escapes much less rajjidly, showing that the external 

 conductibility has been considerably diminished by the friction. 



The distinction which I have established between these two kinds 

 of conductibility accounts for the following facts: — If a glass tube 

 be held in the hand and rubbed against the copper stem of an 



