548 Introduction to the Study of Science 



with sea water and soap ? A great deal of well water is almost 

 as hard as that of the ocean. In some sections of the country, 

 where the well water is hard, cisterns are often used to collect 

 and conserve rain water for household uses. Hard water, how- 

 ever, can be softened by heat or by the addition of such chemical 

 substances as carbonate of soda (sal soda), caustic soda, borax, 

 and ammonia. Many washing powders are mixtures of soap 

 and soda or caustic soda, and such mixtures both soften the 

 water and combine with grease and dirt, facilitating their re- 

 moval. From a survey of the numerous washing compounds 

 on the market and of what they are advertised to accomplish, 

 it would appear that the warfare against dust is of vital con- 

 cern to every one. That dust, moreover, is somehow dangerous 

 to health and life may be inferred from the large number of 

 articles on the market for cleansing our bodies and for the re- 

 moval of dust from the more delicate membranes of nose, 

 mouth, and throat. 



Materials in dust. It is astonishing to note the various 

 kinds of materials contained in dust. If one should select a 

 limited area of the street and list the materials in process of 

 becoming dust, one would discover enough to disturb the usual 

 complacent attitude toward dust and dirt. There are frag- 

 ments of hair, cells from skins and bodies of animals and men, 

 soot, bits of cloth, wood, paper, stone, steel and other metals, 

 tobacco, filth of various sorts, sputum, all being mixed and 

 ground into a more or less impalpable powder to be diffused 

 through the air with every breeze. This should be supple- 

 mented with the fact that many a cigar fragment and the 

 sputum and droplets of moisture in speaking, coughing, and 

 sneezing may come from persons who are victims of infectious 

 disease. 



272. What dust may carry. It is a well-established fact 

 that microbes of diphtheria, measles, whooping cough, in- 

 fluenza, pneumonia, tuberculosis, and epidemic meningitis 

 are expelled in just this manner. These may float about in 



