VOL. LXXVIII.] PHILOSOPHICAL TRANSACTIONS. 505 



that is 57°. It is observable, that in the latter there are 2 causes concur in pro- 

 ducing the effect, the liquefaction both of the snow and salt; but in the ex- 

 periment just mentioned the liquefaction of the salts only. Vitriolated natron, 

 after it had given out its water of crystallization by exposure to the atmosphere, 

 produced no change of temperature by solution in the diluted nitrous acid, but 

 during solution in water produced heat, as did also the mineral alkali. 



XXVI. A Description of an Instrument which, by the turning of a Winch, pro- 

 duces the two States of Electricity without Friction or Communication with the 

 Earth. By Mr. fVilliam Nicholson, p. 403. 



Plate 6, fig. 3, represents the apparatus supported on a glass pillar 64- inches 

 long. It consists of the following parts. Two fixed plates of brass, a and c, 

 are separately insulated and disposed in the same plane, so that a revolving plate 

 B may pass very near them, without touching. Each of these plates is 2 inches 

 in diameter; and they have adjusting pieces behind, which serve to place them 

 accurately in the required position, d is a brass ball, also of 2 inches diameter, 

 fixed on the extremity of an axis that carries the plate b. Besides the more es- 

 sential purpose this ball is intended to answer, it is so loaded within on one side, 

 that it serves as a counterpoise to the revolving plate, and enables the axis to 

 remain at rest in any position. The other parts may be distinctly seen in fig. 4. 

 The shaded parts represent metal and the white represent varnished glass, on is 

 a brass axis, passing through the piece m, which last sustains the plates a and c. 

 At one extremity is the ball d before-mentioned; and the other is prolonged by 

 the addition of a glass stick, which sustains the handle l and the piece gh sepa- 

 rately insulated, e, f, are pins rising out of the fixed plates a and c, ,at unequal 

 distances from the axis. The cross-piece gh, and the piece k, lie in one plane, 

 and have their ends armed with small pieces of harpsichord-wire, that they may 

 perfectly touch the pins ef in certain points of the revolution. There is also a 

 pin I, in the piece m, which intercepts a small wire proceeding from the revolving 

 plate B. 



The touching wires are so adjusted, by bending, that when the revolving plate 

 B is immediately opposite the fixed plate a, the cross-piece gh connects the .2 

 fixed plates, at the same time that the wire and pin at i form a communication 

 between the revolving plate and the ball. On the other hand, when the revolv- 

 ing plate is immediately opposite the fixed plate c, the ball becomes connected 

 with this last plate, by the touching of the piece k against f; the 2 plates, a 

 and B, having then no connection with any part of the apparatus. In every other 

 position the 3 plates and the ball will be perfectly unconnected with each other. 

 Mr. Cavallo's discovery, so well explained in the last Bakerian lecture, that 

 the minute differences of electrization in bodies, whether occasioned by art or 



VOL. XVI. 3 T 



