Intelligence and Miscellaneous Articles. 157 



Thus the increase with the temperature ia very considerable. The 

 linear coefficient of expansion for the temperature t can be put 



0-000061+ 0-00000076 U 



Two strips of ebonite and sheet zinc, when ri vetted together, curve 

 very materially with even a moderate heating. A thin strip of ivory 

 20 centims. in length, cemented by means of isinglass to a similar 

 one of ebonite, even without index, forms a delicate thermometer, 

 seeing that its free end moves through several millimetres for one 

 degree. The curvature in consequence of unequal expansion may 

 be most simply demonstrated by the aid of a mere plate of ebonite ; 

 for, owing to its bad conductivity, it curves considerably when rapidly 

 heated on one side. 



The solid expansion of ebonite is, from the above numbers, at 0° 

 equal to that of mercury ; at higher temperatures it is still greater. 

 It is possible that other kinds expand still more, so that as a curio- 

 sity a mercurial thermometer might be constructed whose scale 

 sunk on being heated. 



The great expansion may possibly be connected with the pro- 

 portion of sulphur which ebonite contains : Kopp found* for the 

 coefficient of sulphur 0*000061 at 30°. On the other hand, the 

 contrast to soft caoutchouc is very remarkable. 



I will mention one fact which was observed in the observation 

 of expansion. The bar of ebonite, which was about a centimetre in 

 thickness, after being heated required a considerable time before it 

 assumed a constant length. Although the bad conductivity is 

 doubtless the principal cause of this, I imagine that another phe- 

 nomenon is also at work. Like the elastic change of form, the ex- 

 pansion by heat may also not take place instantaneously, but con- 

 tinue itself after the change of temperature, gradually becoming 

 weaker. A few observations of Matthiessen's with glass rods 

 (Poggendorff 's Annalen, vol. cxxviii. p. 521) seem to point in this 

 direction ; probably this thermal afteraction, like the elastic, occurs 

 in an eminent degree in organic substances. — Poggendorff 's Au- 

 nalen, No. 8, 1873. 



ON THE DISCHARGE OF ELECTRIFIED CONDUCTORS. 

 BY J. MOUTIER. 



Equilibrium of electricity at the surface of a system of conduc- 

 tors results, as Poisson has shown, from the following condition : — 

 The resultant of the actions exerted by the different electric layers 

 on any point in the interior of the conductors must be no action. 

 If by m be designated one of the electric masses, affected by a sign 

 accordiDg to its nature, by r its distance from a point situated 

 within the conductor, the function 



r 



* Pogg. Ann. vol. lxxwi. p. 156. 



