THE HABITATION AND THE ATMOSPHERE. 171 



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. Bouchar- 

 dat, remarking in his "Treatise on Hygiene" on the exposure to 

 which the tenement population are subjected in wind-penetrated Man- 

 sard-roofs and in damp basements, adds that the commissioners of 

 unhealthy dwellings are w r rong when they rank overcrowding and 

 uncleanliness among the worst sources of danger. 



Dr. Pettenkofer calculates that a house having a cellar and base- 

 ment 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. 



Aeration 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, permeabil- 

 ity, 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 frac- 

 tion of the whole volume. According to Hunt, the decimal of po- 

 rosity is from 0'07 to 0*20 for some sandstones, from 0*06 to 0*14 for 



