PRESERVATIVES AND DISINFECTANTS 39 



candies are often dipped in varnish to prevent the ac- 

 tion of the air. Formalin is sometimes put into milk 

 to prevent souring. No food so treated is good to eat. 



The best and most harmless chemical preservatives 

 are those discovered long ago. They are common salt, 

 vinegar, spices, and wood smoke which is used for pre- 

 serving meat. Spices, however, should be used sparingly. 



Wood is protected by such compounds as varnish and 

 paint. Linseed and poppy-seed oils are the best. Tur- 

 pentine is used to make the paint dry rapidly. Iron 

 and lead oxides are used, but the lead paints are the best 

 if used with good linseed oil. The oil is a preservative, 

 and the thin coat of lead and dried oil on the surface 

 prevents water and air from entering the wood. Rail- 

 road ties and street paving-blocks are soaked in an oily 

 solution of creosote. Creosote is a poison and so prevents 

 the growth of any organisms in the wood. Iron and 

 other metals when exposed to the air and moisture are 

 oxidized and form rust. This is prevented by covering 

 them with lead paint or coal tar. The coal tar is used on 

 bridges made of steel, as it is a- very cheap preservative. 



QUESTIONS AND EXERCISES 



1. Examine all kinds of wooden posts and determine where they 

 decay most rapidly. Explain. How can decay be prevented? 



2. Make a list of the diseases in your community caused by 

 germs. What remedies are used for some of them? 



3. Name some good disinfectants and tell how to use them. 



4. Moisten a small piece of bread, roll it a yard or more on the 

 floor, keep it moist and at room temperature for three or four days, 

 and observe what happens. Explain. 



5. Are your parents careful about the kind of preserved foods 

 which they buy? Why? 



