390 WELLS'S NATURAL PHILOSOPHY. 



Upon what doe8 "^^S. The intensity of the electric light de- 

 the '"'^efec^ric P^nds both upon the density of the accumu- 

 ught depend? Jated electricity, and the density and nature of 

 the aerial medium through which the spark passes. 



Thus, the electric light, in condensed air, is very bright, and in a rarefied 

 atmosphere it is faint and diffusive, like the light of the aurora borealis ; in 

 carbonic acid gas the light is white and intense ; it ia red and faint in hydro- 

 gen, yellow in steam, and green in ether or alcohol. 



„ , If, by means of an air-pump, the air is exhausted from » 



Row may the , ' ,. , , , , , , • , „• 



auroral light be long cyhndncal tube closed at each end with a metallic cap, 



imitated f ^^^ ^ current of electricity passed through it, an imitation of 



the appearance of the aurora borealis is produced. When the exhaustion of 



the tube is nearly perfect, the whole length of the tube will exhibit a violet 



red liglit. If a small quantity of air be admitted, luminous flashes will be seen 



to issue from points attached to the caps. As more and more air is admitted, 



the flashes of hght which glide in a serpentine form down the interior of the 



tube will become more thin and white, until at last the electricity will cease 



to be diffused through the column of air, and will appear as a gUmmering 



light at the two points. 



759. The crackling noise, or sound which is produced 

 by the electric discharge, is attributed to the sudden dis- 

 placement of the particles of air, or other medium 

 through which the electric fluid passes. 



760. The electric shock, or convulsive sensation occa- 

 sioned by the passage of the electric fluid through the 

 body of a man, or animal, is supposed to arise from a 

 momentary derangement of the organs of the body, ow- 

 ing to an imperfection, or difierence in the conducting 

 power of the solids and fluids which compose them. 



If this derangement does not exceed the power of the parts to recover their 

 position and organization, a convulsive sensation is felt, the violence of which 

 is greater or less according to the force of electricity and the consequent de- 

 rangement of the organs ; but if it exceeds this limit, a permanent injury, or even 

 death, may ensue. 



What are the "^61. lu the proccsscs hithcrto described 

 TgTnts in"nl! clcctricity has been developed by friction. In 

 eiectric^ry?'"^ uaturc the agents which are undoubtedly the 

 most active in producing and exciting elec- 

 tricity, are the light and heat of the sun's rays. 



The change of form or state in bodies is also one of the 

 most j)owerful methods of exciting electricity. 



