3 l 2S ELECTRICITY. 



The sensibility of this instrument is very surpriz- 

 ing. The brush of a feather, the throwing of 

 chalk, hair-powder, or dust, against its cap, evince 

 strong signs of electricity. The electricity of va- 

 pour is elegantly shown by pouring a tea-spoonful 

 of water on an ignited coal, placed in a metallic 

 cup upon the cap of this electrometer. 



This instrument is also used for distinguishing 

 the kind of electricity possessed by any body. For 

 when the body is placed upon the cap, so as to 

 make the gold leaves diverge, if a stick of sealing- 

 wax be brought near to it, and the leaves diverge 

 still more, then the electricity is negative ; but if 

 they collapse, then the electricity is positive. The 

 reason of this is, that the electricity of sealing-wax 

 being negative, more of the same kind of electri- 

 city is added, if the electricity of the body under 

 examination be also negative; but if it be positive, 

 then it is neutralized by the negative electricity of 

 the sealing-wax. 



Of the Electric Spark. 



When a piece of metal, or any other good con- 

 ducting substance, is presented to the charged 

 prime conductor, the electric matter will pass with 

 violence from the one to the other : an electric 

 spark, with the appearance of fire, will be seen 

 darting between them; and a slight report, which 

 is usually compared to a snapping noise, will be 

 heard. 



When any person stands upon the stool with feet 

 made of glass (such as is represented Fig. 70> and 

 takes in his hand a chain fastened to the prime 



