Microorganisms in Relation to Man 547 



but more or less filled with microscopic dust particles. The 

 fine, almost invisible particles often fill eyes, nostrils, mouth, 

 and throat, and occasion at least temporary discomfort and 

 pain. Dust accumulation in homes, offices, school buildings, 

 and factories requires a great deal of human ingenuity and labor 

 to effect its removal. It adheres to walls and floors, rugs and 

 carpets, furniture and clothing, and accumulates with astonish- 

 ing rapidity. It seems to be present always and everywhere, 

 and to demand unceasing efforts to prevent its insidious con- 

 quest of human comfort and even health and life. 



Means of removing dust. The methods devised to remove 

 dust even temporarily from buildings suggest the seriousness 

 and magnitude of the problem. The common broom, the age- 

 long weapon of attack upon dust and dirt, the feather duster, 

 brush and dust cloth, carpet sweeper, dustless duster, chemical 

 preparations for collecting and holding dust instead of allowing 

 it to be scattered in the air, and vacuum cleaners of varying 

 efficiency are some of the means employed. This does not 

 complete the list, for water, the most abundant and universal 

 cleanser in nature, must be included ; and with water a score 

 or more of chemical substances used to increase its cleansing 

 properties. One should note some of the ways in which water 

 is utilized to conquer dust. Floors, streets, and highways are 

 often sprinkled, unless the use of oil has made it unnecessary. 

 Water is used for washing ail durable parts of the house, for 

 washing clothes, utensils, and so on indefinitely. It may be 

 used cold or hot, or in the form of steam. 



Hard and soft water. Many difficulties may be presented 

 in the use of water as a cleanser. Water varies greatly in 

 quality because of the different kinds of rock materials through 

 which it may have passed (see page 196), and which it in some 

 measure dissolves away and retains in solution. It may, as 

 is commonly stated, be hard or soft in varying degrees. Hard 

 water is unsatisfactory for cleansing purposes at least, and use- 

 less with ordinary soaps. Did you ever try to wash anything 



