AT a date at least as remote as 600 years B.C. the Greek philosophers were acquainted with a curious little fact to which the modern science of electricity owes its name. They knew that a piece of amber (ηλɛκтρоυ) when rubbed against some suitable substance acquired a temporary attractive power, in virtue of which it became capable of lifting and holding light objects, such as dry leaves or pieces of straw. But another remarkable effect which often attends the friction of amber was for many centuries altogether overlooked. In A D. 1708 it was first noticed by Dr. Wall that a piece of strongly excited amber emitted sparks, which were accompanied by crackling sounds, and these he had the sagacity to compare to thunder and lightning.