50 ^ ELECTRICITY. 



the velocity of the current. A paratonnere will pro- 

 tect from lightning a circular space whose radius is 

 twice that of the height of the conductor. Six*y 

 feet in circumference requires a rod of fifteen feet. 



Electrical attractions and repulsions disappear in 

 vacuo, but that of currents still continues. 



Ancient Electricity. 



Writing concerning certain gems of a red colour, 

 Pliny says, " Et inter has invenio differentiam ; 

 imam, quae purpura radiat ; alteram, quse cocco ; a 

 sole excalefactas, aut digitorum adtritu, paleas et 

 chartarum folia ad se rapere" 



Positive and negative Electricity, 



In the early stages of this science, electricians 

 concluded that excited glass acquired more than its 

 natural quantity of the electric fluid by friction, and 

 that excited wax, or sulphur, or the insulated rubber, 

 lost part of it ; called the first positive, and the last 

 negative electricity. Berzelius divides all substances 

 whatever into two great classes ; viz. the electro- 

 positive and electro-negative. 



Vitreous and Resinous Electricity. 



At present the electricity of glass is called vitreous 

 or positive electricity, and that of sealing-wax resinous 

 or negative electricity. 



Electric Tastes. 



Positive and negative electricity may be readily 

 distinguished by the taste, on making the electric 

 current pass to the tongue by means of a point. 

 The taste of the positive electricity is acid ; that of 

 the negative electricity more caustic, or, as it were, 

 alkaline. 



