i 7 2 THE POPULAR SCIENCE MONTHLY. 



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. Marker 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 men- 

 tioned would stand sandstones, rough stones, limestones, brick, cal- 

 careous tufa, and adobe. Adobe has been found to be twice as per- 

 meable 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, 

 beton, hand-made bricks, green sandstone, molded plaster, oak-wood, 

 and enameled bricks. Plaster is extremely compact, and little favor- 

 able 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 im- 

 purities 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, how- 

 ever, 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 percept- 

 ible to the smell and provoke the uneasiness which is usually attributed 



