ARCHITECTURAL history has many aspects. Ideas and theories, points of view and programmes can have real importance even when, as with the Picturesque and the earlier stages of the Gothic Revival, most of the buildings which derive from them or follow their prescriptions are lacking in individual distinction. Volume of production is also significant; the disproportion between the previous chapter and the four that precede it expresses fairly accurately the difference in the amount of building in the first half of the century belonging, at least by a broad definition, to the rubric of Romantic Classicism and the very much smaller amount—up to 1840 at least and outside England—that can be considered essentially Picturesque or programmatically Neo-Gothic. But the history of architecture must include the history of building as a craft or technic; sometimes the story of technical development is—or has appeared to posterity to be—more important than any other aspect of a particular historical development. Such has been the case until quite lately with the rise of the Gothic in the twelfth century in France; it has also seemed true in varying degree for the nineteenth century to many historians and critics. The Industrial Revolution induced a parallel but gradual revolution in building methods; even today, after two hundred years, the potentialities of that revolution have not been fully actualized. The technical story, particularly as it concerns the structural use of ferrous metals, first cast iron,[146] next wrought iron, and then steel, begins well before 1800. There has already been occasion to mention, in passing, technical innovations in various edifices where those innovations had a determinant effect on the total architectural result. But it is worth while, partly for the intrinsic interest of the subject, partly as preparation for subsequent technical developments of great importance later in the nineteenth and in the twentieth century, to go back to the beginning and to recount sequentially the episodes in the rise of iron as a prime building material, as also to touch at least on the concurrent use of other ‘fireproof’ materials and the vastly increased exploitation of glass. This sequence of episodes reaches a real culmination in the fifties with the construction of a considerable number of ‘Crystal Palaces’, first in London and then all over the western world, edifices that were almost entirely of iron and glass.