XXIV. The reader will observe that, up to the last expansion of the penetrations, the stone-work was necessarily considered, as it actually is, _stiff_, and unyielding. It was so, also, during the pause of which I have spoken, when the forms of the tracery were still severe and pure; delicate indeed, but perfectly firm. At the close of the period of pause, the first sign of serious change was like a low breeze, passing through the emaciated tracery, and making it tremble. It began to undulate like the threads of a cobweb lifted by the wind. It lost its essence as a structure of stone. Reduced to the slenderness of threads, it began to be considered as possessing also their flexibility. The architect was pleased with this his new fancy, and set himself to carry it out; and in a little time, the bars of tracery were caused to appear to the eye as if they had been woven together like a net. This was a change which sacrificed a great principle of truth; it sacrificed the expression of the qualities of the material; and, however delightful its results in their first developments, it was ultimately ruinous. For, observe the difference between the supposition of ductility, and that of elastic structure noticed above in the resemblance to tree form. That resemblance was not sought, but necessary; it resulted from the natural conditions of strength in the pier or trunk, and slenderness in the ribs or branches, while many of the other suggested conditions of resemblance were perfectly true. A tree branch, though in a certain sense flexible, is not ductile; it is as firm in its own form as the rib of stone; both of them will yield up to certain limits, both of them breaking when those limits are exceeded; while the tree trunk will bend no more than the stone pillar. But when the tracery is assumed to be as yielding as a silken cord; when the whole fragility, elasticity, and weight of the material are to the eye, if not in terms, denied; when all the art of the architect is applied to disprove the first conditions of his working, and the first attributes of his materials; _this_ is a deliberate treachery, only redeemed from the charge of direct falsehood by the visibility of the stone surface, and degrading all the traceries it affects exactly in the degree of its presence.