identical with the quantity of chemical affinity; and he was thus led to the clearest view of the truth towards which he and his predecessors had so long been travelling, that electrical and chemical forces are identical[18\5]. [Note 13\5: _Hist. Ind. Sc._ b. xiv. c. ix. sect. 1.] [Note 14\5: See Faraday's Historical Sketch, _Researches_, 481-492.] [Note 15\5: Art. 524.] [Note 16\5: In 1834. Eleventh Series of Researches. Art. 662.] [Note 17\5: _Hist. Ind. Sc._ b. xiv. c. ix. sect. 2.] [Note 18\5: Arts. 915, 916, 917.] 7. It will, perhaps, be said that this beautiful train of discovery was entirely due to experiment, and not to any _à priori_ conviction that co-existent polarities {379} must be connected. I trust I have sufficiently stated that such an _à priori_ principle could not be proved, nor even understood, without a most laborious and enlightened use of experiment; but yet I think that the doctrine, when once fully unfolded, exhibited clearly, and established as true, takes possession of the mind with a more entire conviction of its certainty and universality, in virtue of the principle we are now considering. When the theory has assumed so simple a form, it appears to derive immense probability (to say the least) from its simplicity. Like the laws of motion, when stated in its most general form, it appears to carry with it its own evidence. And thus this great theory borrows something of its character from the Ideas which it involves, as well as from the Experiments by which it was established. 8. We may find in many of Mr. Faraday's subsequent reasonings, clear evidence that this idea of the Connexion of Polarities, as now developed, is not limited in its application to facts already known experimentally, but, like other ideas, determines the philosopher's researches into the unknown, and gives us the _form_ of knowledge even before we possess the _matter_. Thus, he says, in his Thirteenth Series[19\5], 'I have long sought, and still seek, for an effect or condition which shall be to statical electricity what magnetic force is to current electricity; for as the lines of discharge are associated with a certain transverse effect, so it appeared to me impossible but that the lines of tension or of inductive action, which of necessity precede the discharge, should also have their correspondent transverse condition or effect.' Other similar passages might be found. [Note 19\5: Art. 1658.] I will now consider another case to which we may apply the Principle of Connected Polarities.