HOUSEHOLD CHEMISTRY 



95 



lodged between them? What is the nature of the yeast 

 used in bread-making? What is secured through its use? 

 What effect on the yeast has baking the bread ? 



4. Where soil is open and porous enough for free access 

 of the oxygen of the air, and at the same time sufficiently 

 moist, what chemical changes favorable to plant growth 

 may occur (a) in the humus of the soil ; (b) through the 

 agency of nitrogen-fixing bacteria ? 



5. What very largely is the "dust" that accumulates on 

 the floors and furnishings of our homes? Aside from its 

 irritating effects on the 



air-passages and in the 

 lungs, what positive dan- 

 ger is there in breathing 

 it? Where besides in 

 our homes is there dan- 

 ger from breathing dust ? 

 What is meant by infec- 

 tion ? What diseases or- 

 dinarily find their way 

 into the body as germs in 

 the air we breathe ? 



6. What precautions 

 should be taken to avoid 

 filling the air of rooms 

 with dust when sweep- 

 ing and when dusting? 



What adds to the dust nuisance in ^schoolrooms ? What is 

 a common ailment of workers in factories where the air is 

 filled with dust from wood, metal, or cloth? 



44. Use a damp cloth, or oiled rag, 

 when dusting. 



