636 LUMINOUS EFFECTS OF THE DISCHARGE.* 



other gases the colour of the spark is different. Thus 

 the spark has a beautiful carmine red colour if the ends 

 of the conductors between which the discharge takes 

 place are completely surrounded by hydrogen. The 

 mode of observing sparks in various gases will be ex- 

 plained further on; in the meantime it may be stated 

 that to the luminous air there are in each case super- 

 added particles of the metals or other substances 

 between which the spark passes. This is especially the 

 case if the discharge is very powerful, and the metals 

 are very near to each other. The vaporised particles 

 of these metals, although their quantity is extremely 

 minute, give out sufficient light for observing dis- 

 tinctly the spectrum characteristic of each substance. 



The greater the tension which electricity possesses, 

 the greater is the distance through which the dis- 

 charge can take place through the non-conducting 

 intervening air, and the longer therefore are the sparks. 

 But the length of .the spark does riot depend solely on 

 the electrical tension, but also on the density of the in- 

 tervening air. Less dense air is more easily traversed 

 by electricity than denser air, and, hence, the tension 

 being the same, the spark is longer in rarefied air. 



An imperfect barometer, in which the space above the 

 mercury is not a perfect vacuum, is well adapted for 

 observing the passage of electricity through very rare- 

 fied air, provided a wire is inserted at the top, through 

 the side of the tube, for conducting the electricity. 

 When the knob of a charged Leyden jar is brought near 

 to the projecting end of the wire,, while the external 

 coating is connected by a chain with the mercury in the 

 cistern, the discharge takes place through the whole 



