These spaces are filled with the concrete at a temperature of about 80 deg.  C. The tracery and the bas-reliefs impressed on the walls are obtained by means of patterns embossed or marked upon thinner sheets placed inside the metallic frames.  The hardening is effected partly by sudden cooling, partly by the application of electricity under great hydraulic pressure.  The flat roof is constructed in the same manner, the whole mass, when the fluid concrete is solidified, being simply one continuous stone, as hard and cohesive as granite.  Where a flat roof would be liable to give way or break from its own weight, the arch or dome is employed to give the required strength, and consequently all the largest Martial buildings are constructed in the form of vaults or domes.  As regards the form of the building, individual or public taste is absolutely free, it being just as easy to construct a circular or octagonal as a rectangular house or chamber; but the latter form is almost exclusively employed for private dwellings.  The jewel-like lustre and brilliancy I have described are given to the surfaces of the walls by the simultaneous action of cold, electricity, and pressure, the principle of which Esmo could not so explain as to render it intelligible to me.  Almost the whole physical labour is done by machinery, from the digging and mixing of the materials to their conveyance and delivery into the place prepared for them by the erection of the metallic frames, and from the erection to the removal of the latter.  The translucent material for the windows I have described is prepared by a separate process, and in distinct factories, and, ready hardened and cut into sheets of the required size, is brought to the building and fixed in its place by machinery.  It can be tinted to the taste of the purchaser; but, as a rule, a tintless crystal is preferred.  The entire work of building a large house, from the foundation to the finishing and removal of the metallic frames, occupies from half-a-dozen to eighteen workmen from four to eight days.  This, like most other labour in Mars, goes on continuously; the electric lamps, raised to a great height on hollow metallic poles, affording by night a very sufficient substitute for the light of the sun.  All work is done by three relays of artisans; the first set working from noon till evening, the next from evening till morning, and the third from morning to noon.  The Martial day, which consists of about twenty-four hours forty minutes of our time, is divided in a somewhat peculiar manner.  The two-hour periods, of which “mean” sunrise and sunset are severally the middle points, are respectively called the morning and evening zydau.  Two periods of the same length before and after noon and midnight are distinguished as the first and second dark, the first and second mid-day zyda.  There remain four intervals of three hours each, popularly described as the sleeping, waking, after-sunrise, and fore-sunset zyda respectively.