The gradual development of this view was due to the gradual development of new physical concepts which were born in Faraday’s mind and existed there as a poetical vision; but in Maxwell’s mind they appeared as physical quantities having definite quantitative relations to other well-known physical quantities, which a physicist can measure in his laboratory. In every creative physicist there is hidden a metaphysicist and a poet; but the physicist is less apt to persist in his occasional errors as metaphysicist and poet, because the creations of his speculative mind and of his poetical vision can be subjected to crucial experimental tests. Faraday’s “Experimental Researches in Electricity,” published in three thick volumes, looked like very long reading. But my studies at Arran soon convinced me that no reading is long which continually stirs up the interest of the eager reader. Faraday was a pioneer in science, and the descriptions of his explorations read like tales from a new world of physical phenomena, full of poetical visions which his discoveries suggested to his imagination. It must be said, however, that in spite of his wonderful imagination and his free use of it, no investigator ever succeeded better than Faraday in drawing a sharp line of division between the new facts and principles which he had discovered and the visions which his imagination saw in the still unexplored background of his discoveries. For instance, his discovery that a perfectly definite and invariable quantity of electricity is, as we express it to-day, attached to every valency of an atom and molecule, expresses a physical law which his experiments revealed and which he illuminated with all the light of his brilliant intellect. But when this new and precious morsel of the eternal truth had been disclosed by his experiments, then Faraday the scientist stepped aside, and Faraday the poet disclosed his visions about the constitution of matter suggested by what I called at Arran the atomic distribution of electricity in material bodies.