CHAP, iv.] ELECTRICITY AND MAGNETISM. 245 



A perfect vacuum is a perfect insulator no spark will 

 cross it. It is possible to exhaust a tube so perfectly 

 that none of our electric machines or appliances can 

 send a spark through the vacuous space even over so 

 short a distance as one centimetre. 



On the other hand a great increase of pressure also 

 increases the dielectric strength of air, and causes it to 

 resist the passage of a spark. Cailletet compressed dry 

 air at 40 to 50 atmospheres' pressure, and found that 

 even the spark of a powerful induction coil failed to cross 

 a space of '05 centimetre wide. The length of the spark 

 (in air), is also affected by temperature, sparks being 

 longer and straighter through hot air than through cold. 



Flames and currents of very hot air, such as those 

 rising from a red-hot piece of iron, are extremely good 

 conductors of electricity, and act even better than 

 metallic points in discharging a charged conductor. 

 Gilbert showed that an electrified body placed near a 

 flame lost its charge ; and the very readiest way to rid 

 the surface of a charged body of low conducting power 

 of a charge imparted to it by friction or otherwise, is to 

 pass it through the flame of a spirit-lamp. Faraday 

 found negative electrification to be thus more easily dis- 

 charged than positive. Flames powerfully negatively 

 electrified are repelled from conductors, though not so 

 when positively electrified. Sir W. Grove has shown 

 that a current is set up in a platinum wire, one end 

 of which touches the tip, and the other the base, of a 

 flame. 



292. Discharges in Partial Vacua. If the dis- 

 charge take place in glass tubes or vessels from which 

 the air has been partially exhausted, many remarkable 

 and beautiful luminous phenomena are produced. A com- 

 mon form of vessel is the "electric egg" (Fig. 150), a 

 sort of oval bottle that can be screwed to an air-pump, and 

 furnished with brass knobs to lead in the sparks. More 

 often " vacuum tubes," such as those manufactured by 



