109 
It appears, then, that in scarcely more than a year Franklin had 
mastered the theory and practice of electrical science and had be- 
come an investigator. In his letter to Collinson of July 11, 1747, 
he describes an experiment showing ‘‘the wonderful effect of 
pointed bodies both in drawing off and in throwing off the electri- 
cal fire.’’ Moreover, it is in this first scientific letter that he pro- 
pounds his theory of electricity. ‘* We say B (and bodies like 
circumstanced) is electrified fosctively ; A, negatively. Or, rather, 
B is electrified plus ; A, minus. And we daily, in our experiments, 
electrize bodies p/zs or minus, as we think proper. To electrize 
plus or minus, no more needs to be known than this, that the parts 
of the tube or sphere that are rubbed do, in the instant of the fric- 
tion, attract the electrical fire and therefore take it from the thing 
rubbing ; the same parts, immediately as the friction upon them 
ceases, are disposed to give the fire they have received to any body 
that has less.’’ 
In 1745, Kleist,* and, in 1746, Cuneus,} had observed the phe- 
nomena of electrical condensation, and Muschenbroek had con- 
structed the Leyden jar. The experiments of Franklin, made in 
1747, showed that ‘‘ at the same time that the wire and top (inside) 
of the bottle is electrified posctively or plus, the bottom of the bottle 
(outside) is electrified zegatively or minus in exact proportion; 7Z. é., 
whatever quantity of electrical fire is thrown in at top, an equal 
quantity goes out of the bottom.’’ ‘‘ None can be thrown into 
the top when none can get out at the bottom.’”’ . .. . ‘* Again, 
when the bottle is electrified, but little of the electrical fire can be 
drawn out at the top by touching the wire unless an equal quantity 
can at the same time ge¢ zz at the bottom.’’ By these and similar 
experiments he completely analyzed the phenomena in question. 
He continues: ‘‘ So wonderfully are these two states of electricity, 
the pus and minus, combined and balanced in this miraculous bot- 
tle, situated and related to each other in a manner that I can by no 
means comprehend! If it were possible that a bottle should in 
one part contain a quantity of air strongly comprest, and in another 
part a perfect vacuum, we know that the equilibrium would be in- 
stantly restored wzthin. But here we have a bottie containing at 
the same time a A/enum of electrical fire and a vacuum of the same 
fire; and yet the equilibrium cannot be restored between them but 
* Versuche u. Abh. d. Naturf. Gesellsch., Danzig, 1745, Vol. ii, p. 408. 
+ Mémoires del Academie des Sciences, 1746, pv. 2. 
