326 Intelligence and Miscellaneous Articles. 



are bathed, up to a known height, by two conducting masses of 

 concentrated sulphuric acid, into which dip some platinum wires 

 carefully insulated from the sides above the level of the liquid. 

 This apparatus is surrounded by a glass " muff," the air of which 

 is dried by sulphuric acid before commencing the experiments. 



In order to obtain a uniform and slowly variable temperature, 

 the base of the apparatus is inserted, up to a level considerably 

 above that of the acid, in an oil-bath, which is itself surrounded by 

 a sand-bath which can be heated progressively. For the sand-bath 

 a refrigerating mixture can be substituted. The observations were 

 extended to —17° 0. 



If E designates the electromotive force of the pile, p x and p 2 the 

 internal and external radii of the tube, h the height of the liquid, 

 r the specific resistance of glass per cubic centimetre, C the sum of 

 the capacities of the condenser and electrometer, and e the differ- 

 ence of potential communicated to the electrometer (always very 

 small in proportion to E), we have, expressing that the quantity of 

 electricity transmitted through the glass in the time 6 has been 

 employed in charging the condenser, 



27r7tE 



XV. 



Celogfl ^ 



Pi 



Several experiments, made with different heights of sulphuric 

 acid, permit the elimination of the influence of the bottom of the 

 tube, the thickness of which is not the same as that of the sides. 



At the instant of the completion of the circuit the glass tube is 

 at first charged like a condenser. Its interior layers afterwards 

 gradually absorb a certain charge of electricity, necessary for bring- 

 ing them into the definitive state corresponding to the fall of poten- 

 tial established between the surfaces. During this variable state, 

 more or less prolonged according to the nature of the glass, the 

 effects of the charge of the glass are superposed to those of the 

 conductivity. The observations are commenced when the time 

 occupied in charging the glass has assumed a constant value. 



I have also observed that rapid heating determines an apparent 

 increase of conductivity greater than the normal increase : in like 

 manner a rapid lowering of the temperature gives rise to an exag- 

 gerated resistance : but these phenomena quickly disappear, to give 

 place to the normal resistance ; and they are not again produced 

 when the variations of temperature are slow *. 



My observations have hitherto been made on three kinds of glass 

 — common glass (with a base of soda and lime), Bohemian glass, 

 and crystal. 



In all three, raising the temperature produces a rapid increase of 

 conductivity ; the resistance can be expressed by exponential func- 



* These phenomena appear to be due to variations in the dielectric 



power of "-lass under the influence of temperature. 



