This section is from the book "Elements Of The Theory And Practice Of Cookery", by Mary E. Williams. Also available from Amazon: Elements Of The Theory And Practice Of Cookery; A Textbook Of Domestic Science For Use In Schools.
Pure air and pure water we have seen to be simply clean air and clean water. The importance of cleanliness is better understood than ever before, now that scientists have shown the close relation between dirt and disease. The dirt that shows most plainly may not be the most objectionable. A dusty chair is of much less consequence than an unclean dish-cloth.
Two kinds of dust: lifeless and living. - The dirt in houses consists for the most part of dust, both alone and mixed with grease (fatty matter), moisture, and sticky sub-stances. Dust is earth or other matter in particles so fine that it can be raised and carried by the wind. Dust is everywhere present. We see how quickly it gathers on the floor and the furniture; a sunbeam shows us that the air is full of it. This visible dust was for a long time the only kind known about. It has been discovered, however, that mixed with visible dust is another kind, so fine that it can be seen only with a microscope. This invisible dust is composed of tiny plants. When enough plants are growing together they can be seen with the naked eye.
Examine this growth with a magnifying-glass or microscope.
Fig. 3. - Two kinds of mold often found on food.
These microscopic plants are of three kinds, molds, yeasts, and bacteria (singular bacterium, rarely used). We shall learn more about yeast in Chapter IV (Bread. Section 1. Quick Breads; Baking-Powders) (pp. 128-132).
Some kinds of bacteria, if they enter the body where conditions are favorable for their growth, may cause disease. Other kinds cause food to spoil. Bacteria thrive best in dark, damp, moderately warm places, where organic matter is present. Anything that kills bacteria or hinders their growth is called a disinfectant. We shall learn more about bacteria in Chapter III (Eggs And Milk. Section 1. Eggs; Albumin) (pp. 97, 100) and in Chapter X (The Preservation Of Food. Section 1. Microorganisms In Relation To Food) (Sec. 1).
Light promotes cleanliness by revealing dirt and destroying bacteria. Direct sunlight destroys bacteria. Wind may bring dust, but a current of air removes smoke, foul air, and greasy or watery vapors, which combine with dust to deposit an unclean film. Water is the great cleanser, because it is the great solvent. Open windows and blinds and an abundant water-supply are "first aids" to cleanliness.