various dolomites, and 0·30 for the soft Caen limestone and Maltese sandstone. These figures do not, however, permit us to predict the relative permeability of walls into which the stone in question may enter, for that will depend as essentially on the proportion of mortar used and the kind of wash or plaster that is put over the stones, as on the kind of stone employed. It must, then, be determined by direct experiments. These are not wanting. Märker has shown that Walls of brick let more air through than walls of cut sandstone. Arranged in the order of increasing permeability, the building materials here mentioned would stand—sandstones, rough stones, limestones, brick, calcareous tufa, and adobe. Adobe has been found to be twice as permeable as burned brick, having a porosity of sixty per cent, while brick has only twenty-five per cent, by volume. Mr. Lang has made more complete researches on the co-efficient of permeability of different materials, and puts calcareous tufa at the head of his table. Then follow, in the order of decrease, bricks of slag, pine-wood, mortar, béton, hand-made bricks, green sandstone, molded plaster, oak-wood, and enameled bricks. Plaster is extremely compact, and little favorable to natural ventilation.
Paints, washes, and paper-hangings diminish the permeability of walls. The following surfaces are mentioned by Lang, in the order of their increasing effects: whitewash, mastic, glazed papers, common papers, and oil-colors. Common papers are more impermeable than glazed papers, according to Messrs. Putzeys, on account of the greater quantity of starch with which they are impregnated.
Indispensable as is the renewal of the air as a means of preventing moisture in dwellings, it is still more so as a precaution against impurities of every kind that would finally make the atmosphere unfit for respiration. It is, then, important to learn by what sign we may know when an atmosphere is vitiated, and what is the volume of air which a man requires for free breathing in a close room. Normal air, according to the mean of the results of five years of observations at the observatory of Mont Souris, contains about three ten-thousandths by volume of carbonic acid. Immense quantities of this gas are, however, produced in cities by the respiration of the inhabitants and by the fires, but the whole is so rapidly removed by the winds that the atmosphere is not sensibly vitiated by it and it is not necessary to estimate the proportion of carbonic acid, even in the most densely crowded localities, at more than four ten-thousandths.
In an occupied inclosure, like a sleeping-room, a school-room, or a public assembly-hall, the air undergoes a progressive change through the consumption of oxygen and by exhalations from the lungs and the skins of the people; and, unless a sufficient ventilation is kept up, it will in time become unfit for respiration. This will be the case when the impurities with which the atmosphere is charged become perceptible to the smell and provoke the uneasiness which is usually attributed