-1859] WRITINGS OF JOSEPH HENRY. 337 



which a picture as it were of one body has been impressed on 

 another between which the electrical discharge took place. 



Another effect produced by the discharge, and having an 

 important bearing upon the explanation of some of the 

 mechanical results of electricity, is a sudden and violent 

 repulsive energy given to the atoms of air and other sub- 

 stances through which it passes, and which causes them to 

 separate with an explosive violence. 



This may be shown by transmitting a discharge from an 

 electrical battery between two brass balls projecting into the 

 inside of a glass bulb, to the lower side of which is joined 

 an air-tight tube containing a small quantity of water, and 

 opening at the end into a cup of water, the arrangement 

 with the exception of the balls being similar to that of an 

 air thermometer. The moment the discharge takes place, 

 the water will be driven down the tube, exhibiting a great 

 enlargement of the volume of air in the bulb. This experi- 

 ment was communicated by Mr. Kinnersley, of Philadelphia, 

 to Dr. Franklin. The effect at first was attributed to heat 

 produced by the discharge of electricity through the air in 

 the bulb, but although there is heat evolved in this case, (as 

 is proved by the fact that if a number of sparks be passed in 

 succession the water does not return to its first altitude, and 

 thus indicates an increase of temperature,) yet the princi- 

 pal cause is evidently the sudden repulsive energy given 

 to the air at the moment of the passage of the discharge, 

 as may readily be shown by inclosing a thermometer 

 within the bulb. The increase of temperature which this 

 indicates will be far too small to account for the great 

 and sudden expansion produced, A similar exhibition of 

 force is exhibited when a strong discharge of electricity 

 is passed through a vessel (like the one we have described) 

 filled with water. In this arrangement a thick glass bulb 

 may be broken into pieces. 



The mechanical effects produced by lightning must be 

 attributed principally to this cause. When a powerful dis- 

 charge from a cloud passes through a confined space filled 

 with air, and surrounded by partial non-conductors, a tre- 



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