Prof. Buff on the Conductivity of Heated Glass for Electricity, 13 



new, which I have recently made in connexion with this subject, 

 are, I think, worthy of publication. 



Various kinds of glass exhibit, as everybody knows, great dif- 

 ferences in respect to their power of insulation. Potash-glass is 

 in general the best, and soda-glass the worst insulator. One of 

 my hearers, Mr. Matthieson of London, showed me a description 

 of glass, which, when covered on both sides with tinfoil, was 

 completely incapable of assuming a charge. With very slight 

 tension the two electricities crossed the glass and united with 

 each other. By chemical analysis it was found that the glass 

 in question was a soda-glass almost completely free from 

 potash. 



In my experiments I used, for the most part, glass vessels of 

 very small thickness, in the form of chemical test tubes. This 

 glass, although not difficult to fuse, proved at ordinary tempera* 

 tures, and when dry, to be a good insulator. To ascertain the 

 limits at which this property was lost, a tube 39 centims. long, 

 2 centims. exterior diameter, and 0*7 of a millim. in thickness, 

 was filled with water to a height of about 6 centims. Into the 

 water dipped a platinum wire, which remained in connexion with 

 the positively charged conductor of a small electric machine. At 

 a moderate heat no diminution of the insulating power was 

 observed ; but when the temperature reached 40° or 50° C, on 

 touching the exterior surface of the glass with the end of the 

 helix of a very sensitive multiplying galvanometer, the other end 

 of which was in connexion with the earth, a motion of the astatic 

 needle of the instrument could be observed. When the tempe- 

 rature of the liquid approached its boiling-point, the flame which 

 encompassed the tube began to waver, and was repelled from the 

 surface of the glass. When the surface was discharged the 

 wavering of the flame ceased, and at the same time the electro- 

 meter pendulum placed upon the conductor of the machine sunk, 

 although the machine was turned persistently, thus showing that 

 the electricity passed speedily through the glass. By approach- 

 ing the knuckle, sparks were obtained. A battery of 12 of Bun- 

 sen^ s elements was not sufficiently strong to force a current 

 through the glass heated to 100° C. 



To obtain a higher temperature, the water was poured out and 

 its place taken by mercury. In heating the latter to its boiling- 

 point, the glass became such a good conductor, that, by approach- 

 ing the knuckle, sparks an inch in length could be drawn from 

 it, the insulated flame of the spirit-lamp being at the same time 

 blown aside by the repulsion. At a temperature of 220° or 

 230° C, the touching of the surface of the glass by a platinum 

 wire was sufficient instantly to discharge the prime conductor. 

 The current of a single Bunsen's element was sensibly conducted, 



