1 8 Prof. Buff on the Conductivity of Heated Glass for Electricity. 



It was sufficiently powerful to deflect the needle of the tangent 

 compass^ after the removal of the battery, when the coatings were 

 connected with the ends of the helix of the instrument. A de- 

 flection increasing with the temperature of the glass was thus 

 remarked, although the tension must have been diminished at 

 the higher temperatures. At ordinary temperatures, where the 

 tension was greatest, no action whatever was produced upon the 

 needle. 



This decrease of the intensity of the charge, and the simulta- 

 neous increase of the quantity of the electricity conducted from 

 the surfaces of the glass, is quite irreconcileable with the notion 

 of a charge similar to that of the Leyden jar, or a condensation 

 of the electricity on the opposite surfaces of the glass. The effect 

 is a natural and necessary consequence of the assumption, that 

 the glass softened by heating, like a liquid conductor under the 

 same circumstances, has endured a polarization, that is, a che- 

 mical alteration of its surface. 



When the mercury on both sides of the glass had reached its 

 boiling-point, a polarization could be obtained of sufficient 

 strength to produce a deflection of 50° or 60°. This action 

 could be produced by the polarizing powers of a battery of three 

 or four Bunsen's elements, but in a much shorter time with a 

 more powerful battery. 



When the polarization is strongly developed, it persists with 

 great obstinacy, and in all probability the change on which it 

 depends penetrates to some depth into the glass. The following 

 experiment will illustrate this. The glass tube, polarized when 

 the mercury was at its boiling-point, was permitted to cool and 

 then freed from its metallic coatings. The internal and external 

 surfaces were cleansed with nitric acid, washed with distilled 

 water, and dried over the flame of a spirit-lamp. When after- 

 wards the glass tube was immersed in mercury, as before, and 

 partially tilled with the same liquid metal, on heating the 

 mercury again to its boiling-point, and connecting the coatings 

 and the galvanometer wire, a deflection of 30° in the same direc- 

 tion as that of the former polarization was obtained, the needle 

 sinking slowly afterwards towards zero*. 



Electric Batteries in which glass plays the part of the moist 

 conductor. 



When in the mercury of the tube, or in the mass which sur- 

 rounded the latter, a quantity of zinc was dissolved, the electric 

 motion was in a determinate direction, and passed in a positive 

 direction (through the connecting wire?) from the pure mercury. 

 This might be shown either electroscopically or by the galvano- 



* See Kohlrausch in the two preceding Numbers of Phil. Mag.—Ens. 



