healthy because the water they contain increases their conductibility, and, consequently, the flow of heat from within outward; and also because evaporation absorbs or neutralizes much heat. M. Bouchardat, remarking in his "Treatise on Hygiene" on the exposure to which the tenement population are subjected in wind-penetrated Mansard-roofs and in damp basements, adds that the commissioners of unhealthy dwellings are wrong when they rank overcrowding and uncleanliness among the worst sources of danger.
Dr. Pettenkofer calculates that a house having a cellar and basement and two stories of five rooms and a kitchen each, would take 800,000 kilogrammes of bricks, and that these would hold about 40,000 kilogrammes of water. The mortar, although less bulky, would hold as much more water. Thus, the entire masonry would hold, in a house just built, 80,000 kilogrammes or eighty cubic metres of water—a quantity which it is by no means easy to drive out. Among the various means that have been devised for quickly drying the walls of newly-built houses preparatory to tenants moving in, only those can be of real effect that depend on the employment of heat combined with an active aeration. The question is wholly one of promoting ventilation. The lower the temperature, the greater the quantity of air that is needed. At 50° Fahr. a cubic metre of air, which may be already supposed to be three fourths saturated, contains seven grammes of vapor, and is only capable of receiving a little more than two grammes more. Thus, nearly 40,000,000 cubic metres of air at 50° will be needed to absorb the 80,000 kilogrammes of water in the masonry. A moderate wind might, it is true, bring this volume of air in contact with the exposed surface in the course of twenty-four hours; but it is evident that the moisture can not be carried off any faster than it can get through the thickness of the wall to the outer surface; and, when this has to be done, the time required for a more or less complete desiccation would be very long. A suitable degree of heating would greatly hasten the drying, provided the air were continually renewed. If, for example, the temperature of the room were raised to 68° Fahr., the effect—depending partly on the increased capacity of the air to absorb vapor, and partly on the greater rapidity of ventilation—would be five or six times as great.
Aëration is thus the sovereign remedy for the moisture of dwelling-houses, and it is favored by the use of porous materials. Viewed with respect to this point, direct determinations of the porosity, permeability, and hygroscopicity of different building materials are of great interest. Messrs. F. and E. Putzeys, in their work on "Hygiene in the Building of Private Houses," have compiled nearly all that has been published on this subject. It appears from their tables that, in the stones most usually employed, the pores occupy an important fraction of the whole volume. According to Hunt, the decimal of porosity is from 0·07 to 0·20 for some sandstones, from 0·06 to 0·14 for
