And, firstly, how shall floors be constructed? Before the “iron period,” when our Washington Capitol, our City Hall, our old Exchange and Custom House were built, the Roman Mediæval vaults only, were used—either of stone or of brick plastered. When the width of a room was too great for one span, granite columns or brick piers were used, as in our old Exchange, now the Custom house. The floors above the vaults were leveled up and paved with flags or marble tiles. As far as grace, strength and absolute relief from the dangers of fire were concerned, this was a perfect system. But now space is demanded; there must be no more heavy piers and no great thickness of floors. We are therefore forced to use a material which, though not combustible of itself, will do little work if exposed to great heat; and in this is seen the great difference between our fire-proof buildings of the brick period and those of the iron period, and the inferior fire-proof qualities of the latter. The problem now is, to use the minimum of brick and the maximum of iron. I think, therefore, it must be conceded that with the best we can do with this material, there is danger; and the problem might be put thus: “Given Iron, make as nearly fire-proof buildings as possible out of it.” What, then, has been done with it thus far? For columns, we have used cast tubes of all shapes and sizes and the wrought-iron pillars of the Phoenix Iron Company; for girders, we have used compound beams of cast-iron, with wrought ties—built up beams of various forms of rolled and plate iron, bolted and riveted together—and common rolled beams, used double; for floor beams we first used deck beams for wide spans and railroad iron for narrow spans; these have now been superseded by the I beam of various sizes. The Rolling Mills now have on their circulars I beams of great dimensions and suitable for girders, but refuse to fill any but large orders; indeed, I believe that only one mill has rollers for beams larger than thirteen inches, while the others will not put up machinery until they get large enough offers. So we are thus far deprived of large smooth beams of one piece, for girders of long span—beams which no one would desire to hide from view, but which might honestly tell their use to every beholder. For supports between beams we have had Peter Cooper’s _terra cotta_ pots and the four inch brick arches. The former are out of use and the latter are almost universally employed. Corrugated iron—first used in the Columbian Insurance building by Mr. Diaper—has also gone out of use. The destruction of the Fulton Bank, a so-called fire-proof building, sealed its fate as far as floors are concerned.[B] We have also had the experiment of stone floors in the American Exchange Bank, by Mr. Eidlitz, and repeated by another architect in the Mutual Benefit Life Insurance Building, at Newark, N. J. The stone slabs, brick arches, and the Parisian floors—of plaster or