534 SPECIAL METHODS OF DISCOVERY. 



It was by making experiments, and studying their 

 effects, that Franklin jvas led to conceive the theory of 

 plus and minus electricity. He found that if two persons 

 stood upon an insulated stool, and one rubbed the cylinder 

 of an electric machine and the other touched it, both 

 became electrified ; and he concluded that the former 

 parted with some of his natural electricity to the cylin- 

 der, and the latter received an excess. He found that 

 a third person, not insulated, could produce sparks by 

 touching either ; and he concluded that the third person 

 parted with electricity to the one and received it from the 

 other. It was by devising and making his experiment 

 with an electric kite, about the year 1749, that he dis- 

 covered atmospheric electricity, and the identity of elec- 

 tricity and lightning. During a thunderstorm on the 

 commons near Philadelphia, he raised a sijk kite by means 

 of a string, with a key at the lower end of the string, from 

 which to draw the sparks, a silken cord being attached to 

 the key to insulate it from the ground. When the string 

 became quite wet and a conductor, he saw its fibres stand 

 out charged with electricity, and he then, by means of his 

 finger, drew a stream of sparks from the key. 



Between the years 1751 and 1762 Canton discovered, 

 by means of experiments, that the kind of electricity 

 developed by friction depends both upon the kind of sub- 

 stance used as a rubber as well as that which is rubbed, 

 and upon the conditions of their surfaces; that a mass 

 of air can be electrified, and that amalgam of tin in- 

 creases the efficacy of the electric machine. By means of 

 actual trial, Beccaria, in 1753, discovered the imperfect 

 electric conducting power of water. JEpinus also, in 

 1759, by heating a tourmaline, discovered that its oppo- 

 site ends were charged with opposite kinds of electricity ; 

 he invented experiments by means of which the kind and 



