12 Prof. Buff on the Conductivity of Heated Glass for Electricity. 



meant, the proposition would be untrue. In the second propo- 

 sition, however, it is the total expansion that is referred to*. 

 Now supposing the conductivity of the bearer to be infinite, that 

 is to say, that the quantity of heat which it receives from the 

 rocker during contact is instantaneously distributed equally 

 throughout its entire mass, then, although the total expansion 

 might be very great, there would be no local expansion at all, 

 and therefore none of the effects in question. The expansion 

 we require is a sudden elevation of the point where the rocker 

 comes into contact with the bearer ; and it is manifest that '^ a 

 rapid communication with the interior^' may, by suddenly with- 

 drawing the heat from the point where it is communicated, 

 almost extinguish the requisite elevation, and thus prevent the 

 vibrations. This appears to be the precise reason why Professor 

 Forbes has failed to obtain the numerous results described in 

 the foregoing pages. His bearers were of such a form that the 

 mass of matter, immediately surrounding the point of contact, 

 quickly abstracted the heat communicated to that point, and 

 thus destroyed the condition upon which the vibrations depend. 

 The success of the experiments described in this memoir depends 

 on the precaution, that the abstraction of heat was prevented, to 

 some extent, by reducing the bearers to laminas and mere spikes ; 

 and the fact that a thin edge gave a better tone than a thick 

 one thus receives a full explanation. These considerations, I 

 think, render it clear that the cause of the superiority of lead 

 assigned by Professor Faraday is by no means an " oversight.'^ 

 On the other hand, it would not be safe to affirm generally, nor 

 has it been affirmed by the philosopher last mentioned, that the 

 less the conducting power the greater will be the effect. In the 

 case of glass and earthenware the vibrations soon come to an 

 end, for the requisite difference of temperature between rocker 

 and bearer, as anticipated by Professor Forbes, soon ceases. 

 Perfect non-conductibility would be just as inefficacious as per- 

 fect conductibility, and the region of practical results lies be- 

 tween these two extremes. 



II. On the Conductivity of Heated Glass for Electricity, 

 By Professor BuFFf. 



IT is known that glass strongly heated assumes the property 

 of conducting electricity. This effect, although often ob- 

 served, has not, however, since the time of Cavendish, been further 

 investigated. A number of observations, some of them totally 



• Seebeck makes use of the same argument. — J. T. 



t Abbreviated from the Ann. der Chetn. und Pharm. vol. xc. p. 257. 



