200 



ORD KELVIN, DR. M, MACLEAN, AND MR. A. GALT, ON THE 



ending in a needle point inside, and connected outside with the insulated terminal of 

 an electric machine, M. By means of an air-pump air is drawn into the canister, 

 where it is electrified by the needle. It passes thence through a few metres of 

 indiarubber pipe, to a 2-metre length of glass combustion tubing, G, 2 centims. 

 internal diameter, heated to a high temperature in a gas furnace, F. The hot air 

 passes on through a length of 3f metres of block-tin piping coiled in a large vessel of 

 cold water, W. The air thus cooled passes through two paraffin tunnels between 

 which is the insulated filter consisting of block-tin pipe with two wire gauzes and a 



Fig. 5. 



plug of cotton wool. There were altogether 10^ metres of tubing between the 

 canister and the filter. The air in the canister is kept electrified by an electrified 

 needle point during an experiment. 



39. Beginning with the glass tube cold, the air gave an electrification at the rate 

 of 14 volts positive for 200 strokes of the pump. On gradually increasing the 

 temperature of the tube the electrification correspondingly diminished to less than 

 3 volts. In cooling, the electrification, now become negative by an accidental reversal 

 in the inductive machine used, increased to 4'5 volts negative. On another occasion 

 5 volts negative were got with the tube cold, decreasing to 2 volts as the temperature 

 was raised, increasing again to 5 volts as the tube cooled. Occasionally irregular 

 results were noted, especially with positively charged air. 



