ON CHEMICAL COMBINATION AND ELKCTRIC DISCHARGE. 489 



discoverer. Hittorf measured the fall of potential in a vacuum tube, not 

 merely between the electrodes but also along the whole length of tube, and 

 found the very remarkable and interesting result that by far the greater 

 part of the total fall in potential occurred close to the cathode. Thus in 

 one of his experiments on hydrogen at a pressure oi about yjrirth of a 

 millimetre of mercuiy he found tliat, of the total fall in potential, about 

 jl occurred close to the cathode, the potential gradient in the rest of the 

 tube only amounting to two or three volts per centimetre. The question 

 immediately suggests itself whether, if we got rid of the electrodes alto- 

 gether, we should reduce the potential difierence to one-sixth or so of the 

 value required when electrodes are used. I have here an experiment which 

 is intended to settle this point ; the apparatus consists of two bulbs con- 

 nected together by an open tube, so that the bulbs are always filled with 

 the same kind of gas at the same pressure. One bulb is without electrodes ; 

 the other, whose diameter is approximately equal to the circumference of 

 the first, is provided with electrodes which are placed at the opposite ends 

 of a diameter ; these electrodes are connected with a wire which makes one 

 turn round the coil which connects the outsides of the two Ley den jars 

 (fig. 1) ; the bulb without electrodes is placed inside this coil ; the total 

 electro-motive force acting round the bulb without electrodes is thus ap- 

 proximately the same as that acting between the electrodes of the other 

 bulb. Setting the Wimshurst machine in action, and gradually increasing 

 the length of the spark until a spark passes, it is found that the discharge 

 begins to appear at about the same time in each of the bulbs, showing that 

 the total electro-motive force required to produce discharge is not very dif- 

 ferent in the two cases, and that the potential difierence required to start 

 a discharge through a given length of gas is not very greatly inci'eased by 

 the presence of electrodes. We may, therefore, conclude that the potential 

 differences measured in the tube with electrodes are primarily connected 

 with work required to split up the gas thi'ough which the discharge passes. 



Resistance of Rarefied Gases. 



Rarefied gases are exceedingly good conductors of electricity when 

 they are acted upon by electro-motive forces sufficiently intense to produce 

 discharges. This is cleai'ly shown by the following experiment. A and B 

 (fig. 2) are two coils in series placed in circuit with the outer coatings of 

 two Leyden jars. In coil A an exhausted bulb is placed ; this bulb serves as 

 a kind of galvanometer, the brightness of the ring in it giving an indica- 

 tion of the current passing through the coil A. The substance whose re- 

 sistance is to be tested is placed in a bulb inside the other coil ; the cur- 

 rents induced in this bulb will, by their inductive action, exert on the 

 primary coil an electro-motive force in the opposite direction to the current 

 in the coil ; this will tend to stop the current, and we shall detect its effect 

 by the diminution in brightness of the discharge in the bullj inside A. 

 The extent of this diminution will give us a clue to the magnitude of the 

 currents induced in the bulb B (fig. 2). I place inside the coil B a 

 bulb containing gas at a low pressure. You notice that the discharge in 

 A is quite extinguished. I now replace this bulb by one of the same size 

 filled with sulphuric acid and water in the proportions for which they con- 

 duct electricity best. You observe that the sulphuric acid in B does not 

 diminish the brilliancy of the discharge in A to anything like the extent 

 the exhausted gas did ; thus the currents passing through the gas are 



