DUST AND DAMPNESS. 109 



This is composed mainly of mineral matter, and is not injurious 

 except as it is irritating. As it sifts into our houses, it is the 

 bane of careful housekeepers. 



It has been known for a great while that there were other con- 

 stituents of dust than mineral matter, and the question what and 

 how much is an important one. Aristotle pronounced dust 

 injurious ; all dry bodies which become damp, and all damp 

 bodies which are dried, he said, engender animal life. After the 

 discovery of the microscope, the theory of animalcula and the spon- 

 taneous generation of them ruled the world. A mixture of flour 

 and water set out in the air will ferment, and a bit of bread or 

 cheese will in a night be covered with mould. The quickness of 

 such growths seems magical, and it was asked how it could be 

 possible unless the germs grew spontaneously, but we now know 

 that it is due to an immense number of exceedingly minute spores. 

 Pasteur, about thirty years ago, showed how dried bodies which 

 became damp engendered life. Pouchet had found that when a 

 tube was heated so as to destroy all germs and then inverted over 

 mercury, organisms were developed in it, but Pasteur in 1864 

 showed that the dust which clung to the mercury and that which 

 was suspended in the air carried the germs of life into the appar- 

 ently sealed tube. He rendered this dust visible by darkening a 

 room and allowing a beam of light to enter through a tiny open- 

 ing. This is the dust of the house, which falls on everything. 

 Everyone knows that dust is visible in a beam of sunlight when 

 none can be seen elsewhere in a room, and the careful old New 

 England housekeeper thought that sunshine brought in dust, and 

 so shut up the best room and kept it dark, and alao damp and 

 musty, which conditions caused the organisms in the air to 

 increase at the rate of many thousands in twenty-four hours. 



It is now well recognized that air everywhere contains the 

 spores of moulds and bacteria, and air free from them is only to 

 be found on the tops of high mountains and perhaps at sea. 

 Many of them are too small to be seen by the microscope. 



The amount of this matter and its effects are important, but we 

 know very little about them, except that these fine particles are 

 everywhere. If a pan of flour and water is set out-doors, enough 

 jeast germs will fall on it to cause it to ferment. In-doors, in 

 ■dwelling-houses and public halls, and wherever people come 

 together, spores are found. They may be divided into moulds 



