And yet these contrasts, strong when we compare extreme cases, fade from our consciousness in the actual use of a mother-tongue. The function makes us unconscious of the instrument, all the more as it is an indispensable and almost invariable one. The sense of euphony accordingly attaches itself rather to another and more variable quality; the tune, or measure, or rhythm of speech. The elementary sounds are prescribed by the language we use, and the selection we may make among those sounds is limited; but the arrangement of words is still undetermined, and by casting our speech into the moulds of metre and rhyme we can give it a heightened power, apart from its significance. A tolerable definition of poetry, on its formal side, might be found in this: that poetry is speech in which the instrument counts as well as the meaning--poetry is speech for its own sake and for its own sweetness. As common windows are intended only to admit the light, but painted windows also to dye it, and to be an object of attention in themselves as well as a cause of visibility in other things, so, while the purest prose is a mere vehicle of thought, verse, like stained glass, arrests attention in its own intricacies, confuses it in its own glories, and is even at times allowed to darken and puzzle in the hope of casting over us a supernatural spell. Long passages in Shelley's "Revolt of Islam" and Keats' "Endymion" are poetical in this sense; the reader gathers, probably, no definite meaning, but is conscious of a poetic medium, of speech euphonious and measured, and redolent of a kind of objectless passion which is little more than the sensation of the movement and sensuous richness of the lines. Such poetry is not great; it has, in fact, a tedious vacuity, and is unworthy of a mature mind; but it is poetical, and could be produced only by a legitimate child of the Muse. It belongs to an apprenticeship, but in this case the apprenticeship of genius. It bears that relation to great poems which scales and aimless warblings bear to great singing--they test the essential endowment and fineness of the organ which is to be employed in the art. Without this sensuous background and ingrained predisposition to beauty, no art can reach the deepest and most exquisite effects; and even without an intelligible superstructure these sensuous qualities suffice to give that thrill of exaltation, that suggestion of an ideal world, which we feel in the presence of any true beauty.