ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 301 



vacuum over mercury, it was ascertained that mortar is practically 

 merely a coarse sieve and permits the rapid and easy passage of gases, 

 that plaster of paris is also highly permeable, 75 per cent compared 

 with mortar ; roman cement permeable to the extent of 25 per cent, 

 and portland and hygienic cement to the extent of about 10 per cent. 

 The rate of diffusion of gases through porous septa is, by Graham's law, 

 in the inverse ratio of the square root of their gravity. If the gases 

 in the earth below the flooring be heavy compared with the air of the 

 room, upward diffusion through the flooring material must be rather 

 slow, unless other apertures for the ingress of outside air are insuffi- 

 cient to supply the draft of fires. When the ground is warm, as in 

 autumn, and contains certain light gases and vapor, there may be 

 considerable aspiration from the ground through the floor into the 

 room. It seems probable that mortar and other porous material would 

 permit the passage or penetration not only of gases, but of microbes, 

 but that good cement would not permit the passage or penetration of 

 microbes to aDy important extent. Asphalt is still better, and effectu- 

 ally shuts out both gases and germs. Coal gas has been known to 

 pass a considerable distance through the earth under frozen ground 

 and to enter a house through the flooring, and there can be no doubt 

 that much ground air enters houses in this way, especially in autumn 

 and winter. A good concrete layer, 4 to 6 inches thick, or asphalt, 

 under every house would do much to diminish diseases caused by 

 ground air. The reduction of two courses of bricks, which would be 

 saved by diminishing the air space between floor and ground, would 

 partly balance the additional cost. 



MECHANICAL VENTILATION IN SCHOOLS. 



From a paper by Professor Carnelley on mechanical ventilation in 

 schools, Sir Henry Eoscoe drew the following conclusions, briefly sum- 

 marized : 



By mechanical ventilation the microorganisms were reduced to one- 

 tenth, the organic matter to one-seventh, and the carbon dioxide to one- 

 half; the temperature was kept higher without draft, and cold drafts 

 were excluded. In badly ventilated schools microbes increase up to a 

 certain point with increase of wall space; in mechanically ventilated 

 schools the microbes decrease with increase of space. Scrubbing or 

 washing floors had no effect in reducing the emission of microbes into 

 the air, and it was found that the infection of a school with these organ- 

 isms takes place very gradually, old schools being much more infested 

 than new buildings. Similar facts have been observed by Miquel as 

 regards houses. It is clear that walls and floors and perhaps ceilings 

 also should be faced with an impervious material, adapted for frequent 

 washing, and without interstices. As regards mechanical ventilation, 

 however, it has not yet been proved that proper natural ventilation can 

 advantageously be superseded. 



