496 . EEPOBT— 1880. 



I had a smaller apparatus, with Volta discs of only about half an inch diameter,, 

 and with gas-tig-ht enclosing case, constructed in the year 1871 ; and I have made 

 many experiments with it, of which I hope soon to publish an account. 



Connections. 



The insulated plate was connected by a stiff brass wire passing through a wide 

 enough hole in the case of the Volta condenser to the electrode of the insulated 

 pair of quadrants. The upper plate was connected to the metal case of the Volta 

 condenser and to the metal case of the electrometer, one pair of quadrants of which 

 were also connected to the case. One of the terminals of the divider, which con- 

 nected the poles of the cell through a graduated resistance coil, was connected to 

 the case of the electrometer, and to the other terminal was attached one of the con- 

 tact wires, which was a length of insulated copper wire having soldered to its outer 

 end a short piece of platinum. The other contact piece was a similar short piece of 

 platinum fixed to the insulated electrode of the electrometer. Hence it will be seen 

 that metallic communication between the two plates was effected by putting the 

 divider at zero and bringing into contact the two pieces of platinum wire. 



Order of Experiment. 



The sliding piece of the divider was put to zero, and contact made and broken 

 and the upper plate raised, and the deflection of the spot of light was observed. 

 These operations were repeated with the sliding piece at different numbers on the 

 divider scale until one was found at which the make-break and separation caused 

 no perceptible deflection. The number thus found on the divider scale was the 

 number of hundredths of the electro-motive force of the cell, which was equal to 

 the contact electric difference of the discs in the Volta condenser. 



[Addenchwi, November 23, 1880. — Since the communication of this paper to the 

 British Association, I have found that a dry platinum disc, kept for some time in 

 dry hydrogen gas, and then put into its position in dry atmospheric air in the 

 apparatus for contact electricity, becomes positive to another platinum disc which 

 had not been so treated, but had simply been left undisturbed in the apparatus. 

 The positive quality thus produced by the hydrogen diminishes gradually, and 

 becomes insensible after two or three days.] 



5. On a method of detertnim,ing without mechanism the limiting Steam- 

 Liquid Temperature of a Fluid. By Professor Sir William Thomson, 

 M.A., F.B.S. 



A piece of straight glass tube — 60 centimetres is a convenient length — is to be 

 filled with the substance in a state of the greatest purity possible. It is to contain 

 such a quantity of the substance that, at ordinary atmospheric temperatures, about 

 3 or 4 centimetres of the tube are occupied by steam of the substance, and the re- 

 mainder liquid. Fix the tube in an upright position, with convenient appliances 

 for warming the upper 10 centimetres of the lengtli to the critical temperature, or 

 to whatever higher or lower temperature may be desired ; and for warming a length 

 of 40 centimetres from the bottom to some lower temperature, and varying its 

 temperature conveniently at pleasure. 



Commence by warming the upper part until the surface of separation of liquid 

 and steam sinks below 5 centimetres from the top. Then warm the lowest part 

 until the surface rises again to a convenient position. Operate thus, keeping the 

 surface of separation of liquid and solid at as nearly as possible a constant position of 

 3 centimetres below the top of the tube, until the surface of separation disappears. 



The temperature of the tube at the place where the surface of separation was seen 

 immediately before disappearance is the critical temperature. 



It may be remarked that the changes of bulk produced by the screw and mercury 

 in Andrews' apparatus are, in the method now described, produced by elevations 

 and depressions of temperature in the lower thermal vessel. By proper arrange- 

 ments these elevations and depressions of temperature may be made as easily, and 

 in some cases as rapidly, as by the turning of a screw. The dispensing with all 

 mechanism and joints, and the simplicity afforded by using the substance to be ex- 



