NOMOS. 15 



having thus diverged, the instrument be carried to 

 the other end. Why there should be any difference, 

 even in degree, will appear presently. 



In the circumstances of the discharge there appears 

 to be a very great difference between the two elec- 

 tricities. Ordinary electricity, as everybody knows, 

 may be discharged at a considerable distance through 

 the air. It is enough to bring the knuckle within 

 the neighbourhood of the charged conductor to 

 receive the spark and shock. With voltaic electri- 

 city, on the contrary, actual contact is necessary 

 to the discharge ; and it is generally necessary to 

 moisten the hands which grasp the conductor be- 

 fore the shock can be felt. The discharge of ordinary 

 electricity, however, is greatly facilitated under cer- 

 tain circumstances, as by making the current pass 

 through heated air, or through a vacuum ; and Dr. 

 Faraday has also shown that the discharge of voltaic 

 electricity is facilitated under the same circumstances. 

 He has shown that voltaic electricity may be dis- 

 charged at a considerable distance across the ex- 

 hausted receiver of an air-pump, or when the poles 

 and the intervening space are heated by a spirit 

 lamp, and that the discharge was interrupted when 

 air was re-admitted into the receiver, or when the 

 heat was removed. Voltaic electricity may also be 

 discharged at some distance when the poles are first 

 joined for some time and then separated. Under 

 these circumstances the poles and the surrounding 

 air become heated by the current while the poles 

 remain together, and this heat acts like the heat 



