234 



ELECTRIC EXPERIMENTS. 



minous effects are best exhibited in the 

 dark. 



(30) If two barometer tubes, united 

 at the top, be filled with mercury and 

 inverted over two cups of mercury, as 

 shown in Fig. 178, a Torricellian vacuum 

 will be formed. When the mercury of 

 one cup is connected with the prime con- 

 ductor and the other with the earth, the 

 upper part of the tube (containing only 

 mercuric and other vapors) is filled with 

 light. The luminosity may be increased 

 by raising the temperature and thus in- 

 creasing the density of the aeriform con- 

 ductor. (A true vacuum will not conduct 

 electricity. ) 



(31.) "Geissler's Tubes" for electric 

 light are sealed glass tubes containing 

 a highly rarefied vapor or gas, with which 

 the tubes were filled before the exhaus- 

 tion. Platinum wires are sealed into the 

 glass at each end, to conduct the elec- 

 tric current. The brilliancy and beauty 

 of the light, the great variety of effects, 

 ^^^ color, and fluorescence, are indescribable. 



They are made in great variety of form and size and filled with 



FIG. 178. 



FIG. 179. 



rarefied vapors and gases of many kinds. A few of the 

 forms are represented in Fig. 179. 



(32.) On the glass table of the universal discharger (Fig. 171) 

 place a piece of wood and bring the knobs of the sliding rods against 

 its ends so that the line joining the knobs shall be in the direction 

 of the fibers of the wood. Through the apparatus thus arranged, 

 discharge a powerful battery. The piece of wood will be torn in 

 pieces. 



(33.) Support a pane of glass upon a glass cylinder, in the axis 

 of which is a pointed conductor which just touches the pane. On 



