between Heat and Static Electricity. 451 



spiral was heated, a marked diminution in the indications of the 

 electroscope was observed. After the apparatus had again 

 attained the normal temperature, the current was sent continu- 

 ously, the rise of the thermometer being noted every minute*, 

 and the brass ball being electified and left unelectrified in alter- 

 nate minutes. The thermometer indicated on the average a 

 rise of one degree when the ball was not electrified and five 

 degrees when it was electrified — thus proving the existence of a 

 downward current of hot air when the ball was electrified. 



Each time the machine was worked the sudden rush of air 

 from the white-hot spiral caused its temperature to be appreci- 

 ably lowered. On filling the vessel with smoke, the upward 

 convection-currents of heated air showed themselves plainly. 

 On working the machine, the sudden downward rush of smoke 

 from the spiral to the ball was very striking. The smoke dis- 

 appeared with surprising rapidity when the ball was electrified ; 

 it was doubtless burnt in passing over the heated spiral. Re- 

 peated experiments proved that the smoke was consumed very 

 much more rapidly when the ball was electrified than when not 

 electrified. 



If heated air be the cause of the discharge, the heated wire 

 will possess no power of discharge through rock-salt. This is 

 actually the case ; the ignited wire may be placed ever so close 

 to a charged Peltier's electrometer without discharge, if a plate 

 of rock-salt be interposed between them ; but remove the salt, 

 and discharge is instantaneous. 



An induction-coil giving a spark ^ inch will give a spark £ 

 inch if the electrodes are at a high temperature. A 1-inch 

 spark becomes 1 \ inch under the same conditions. 



As it might be said that the lengthening of the spark of the 

 coil was due to rarefaction of the air, the following experi- 

 ment was made. Platinum wires were fused into the ends of a 

 short piece of combustion- tube. The spark from the small coil 

 passed through a distance of -5^ of an inch when the glass, 

 except the fused ends, was strongly heated ; the air of course 

 was here almost at the same density. As it is possible to ex- 

 plain the apparent conductivity of glass observed by Dr. Guthrie 

 on the above supposition of air-currents, I sent the spark from 

 the small coil through the heated end of a closed glass tube. I 

 also found that the current from four Grove's cells would pass 

 through \\ inch of heated glass and deflect a galvanometer. 



This power of conductivity is attained at a red heat, and in- 

 creases enormously with slight increase of temperature. Thus, 



* A clock striking minutes with the hand making a complete revolution 

 in the same time was used in all time experiments. 



2H 2 



