198 REroET— 1880. 



dioxide, coal gas, sulpliuric dioxide, &c., a decidedly greater. Lastly, 

 since tlie resistance of a gas to disruptive discharge varied with tLe pres- 

 sure, it was anticipated — also in opposition to the results of Faraday's 

 experiments — that the specific inductive capacity of even the vacuum of 

 an ordinary air-pump must be slightly different from unity, a conclusion 

 also subsequently verified by experiment.' 



The method employed for that investigation which was cariied out in 

 Japan consisted in using two condensers, one an open air condenser of 

 adjustable capacity, the other a closed condenser into which any gas at 

 any pressure could be jjut. The open air condenser was adjusted to have 

 the same capacity as the closed one when the latter was filled with air at 

 the ordinary pressure and temperature ; then the change in capacity of the 

 latter when tbe pi-essure of the air inside was diminished, or when another 

 gas was introduced, could be determined by changing the insulated coat- 

 ings of these two condensers to equal and opposite potentials, discharging 

 thera into one another, and measuring the resultant potential Avith a 

 Thomson's quadrant electrometer adjusted for great sensibility. 



Previously, however, to this, but quite unknown to these members of 

 your Committee, Prof Boltzmann had made a similar investigation, using, 

 however, a different method of experimenting. The results obtained in 

 the two independent investigations for the same gases are placed under- 

 neath side by side, and the fairly close agreement, considering the extreme 

 delicacy of the experiments, make it quite certain that the general bearing 

 of the experinients is correct : — 



Ayrton and Perrv ]5oltzniauii 



Air 1-0000 ' .... 10000 



Vacuum .... 0-9985 .... 0-9994 



Hydrogen .... 0-9998 .... 0-9997 



Coal Gas .... 1-0004 .... 



Marsh Gas .... .... 1-0004 



Carbonic dioxide . . . 1-0008 .... 1 -0004 



.Sulphuric dioxide . . 1-0037 



Tbe gases were at 760 mm. pressure ; the vacuum varied from about 

 10 mm. to somewhat gi"eater pressures. The observation for sulphuric 

 dioxide is given, as it is the highest specific inductive capacity yet ob- 

 tained for any gas. 



The very peculiar behaviour of a good Sprengel A-acuum in resisting 

 the passage of an induction spark led to tbe formation of this Committee, 

 to investigate, with the aid of a grant from the Association, the specific 

 inductive capacity of a far higher vacuum than had been employed in 

 either of the two preceding investigations since Messrs. Ayrton and Perry 

 predicted that such a vacuous space would be found to have a capacity 

 very considerably smaller than if filled with ordinary air. 



The closed condenser in this case consists of five aluminium cylinders 

 39-3 ceiitimetres long, placed concentrically at about ^ centimetre apart, 

 in a glass tube .58'5 centimetres long and 5"5 centimetres in diameter. 

 The second and fourth cylindci-s form the insulated coating, and the first, 

 third, and fifth the earth coating. The cylinders comprising eaeli coating 

 are rigidly connected at each end with a thin platinum rod, and these 

 platinum rods, like the cylinders, do not touch the glass tube, but are held 

 in position by a thin glass rod, one end of which is fused to the platinum 



' ' On tlic Specific Inductive Capacity of Gases,' by Jolm Perry and W. ]-;. Ayrton, 

 Trans. Asiatic Stw. of Japan, Vol. v. part i. p. 116. 



