These mysterious, sub-sensible agents possess certain very definite properties. Their paths are deflected in a magnetic field; they can traverse metallic films; and their investigation in the open, thereby rendered feasible, has shown them to possess photographic efficacy, and the faculty of breaking down electrical insulation; moreover, they transport a negative charge of fixed amount, and have a determinate momentum. They are, then, assuredly no mere pulsations of the ether; unless our senses 'both fail and deceive us,' their quality is material. Material, yet not quite with the ordinary connotation of the term. The most essential circumstance about the cathode-rays is that they remain unmodified by the chemical diversities of the originating gases.[56] A hydrogen tube yields identically the same radiant matter as an oxygen or a nitrogen tube. Here, then, at last we have within our grasp undifferentiated substance--matter not yet specialized, neither molecular nor atomic, matter destitute of affinities, exempt from the laws of combination--matter in its inchoate, and perhaps ultimate form; in a word, the far-sought protyle. Already, in 1879, Sir William Crookes conjectured the infinitesimal missiles propelled from the cathode to be the 'foundation-stones of which atoms are composed.'[57] And in 1886 he pronounced them more decisively to be the raw material of atoms, which, to Sir John Herschel's apprehension, bore the unmistakable stamp of a 'manufactured article.' Nor did his recent commentator refrain from attempting distantly to divine the method of their construction, or from laying his finger on the by-products and residues associated with it,[58] although he felt compelled to relegate the cosmic factory to the edge of the world, where inconceivable things may happen. All this, indeed, seemed, in the late Victorian era, like mounting the horse of Astolfo for a trip to the moon; and sane common-sense pronounced it fantastic enough to 'make Democritus weep and Heracleitus laugh.'[59] But we have since learned from Nature herself some tolerance of audacities. Step by step the new order of ideas has irresistibly come to the front. It owed its origin to Sir William Crookes's skill in producing high vacua, and the consequent development in his tubes of radiant effects. Then, in 1879, universal importance was claimed for them, and matter in the 'fourth state,' by a revival of the dreams of the ancients, expanded into a kind of visionary protyle. Philipp Lenard made the next advance towards its actualization by slipping it, in 1894, through an aluminium window, and watching its behaviour towards ordinary matter. Two years later Röntgen rays made their entry on the scene; and before the end of 1896 Becquerel, hurrying along the track of novelties, came upon the momentous discovery of radio-activity.