While, in recent years, the progress of the science of electricity has been very rapid, few investigations have been made in the old province of frictional electricity. It cannot be doubted, however, that the laws connecting electricity with friction, and with the nature of the substances rubbed, are of great importance; and the acquisition of more detailed knowledge in this department may throw some light on the still imperfect theory of the voltaic cell. Several electricians have expressed an opinion that the development of electricity by friction is only a modification of the development of electricity by contact–that friction is contact in which the number of points which come together is increased by sliding the one substance over the other. But whether friction is a form of contact, or contact a form of friction, or the two co-ordinate to one another, it is interesting to inquire whether the metals can be arranged in an electro-frictional series similar to the electro-contact series; and if so, to observe the relation of the former to the latter.