LIGHTING THE HOME 43 



3. One week later, read again. How much electricity was 

 used in one week? Find out the price of electricity and com- 

 pute the cost for one week. 



Comparison of lights. These different means of lighting 

 may now be compared to see which is the best. From the 

 standpoint of health one point to consider is the effect on 

 the air of the room. In this respect the electric light is the 

 best as it does not affect the composition of the air. All 

 other forms of lighting take oxygen from the air and give 

 off waste products into the air as a result of burning. In 

 those lamps in which there is a complete burning, as the 

 mantle gas lamp and acetylene, only carbon dioxid and 

 water are given off. But in those cases where the burning 

 is not complete, as with the kerosene lamp and ordinary gas 

 burners, besides those gases there are given off others which 

 are injurious in their effect on the health. 



Another point to consider is the effect of the various lamps 

 on the eye. Steadiness is an important requirement. From 

 this standpoint, the electric lamp, the acetylene flame, and 

 the mantle gas lamp are the best, as they give a steady light. 

 Next comes the kerosene lamp, which is fairly steady ; and 

 poorest of all are the candle and ordinary gas burner, which 

 give such a flickering flame as to tire the eye. 



Glass and sunlight. To-day our houses during the day- 

 time are brightly lighted by the sunlight that comes through 

 the window panes. But before glass was made, houses were 

 dark and clingy, because people used oiled paper and isin- 

 glass in their windows. Glass to-day is so common and cheap 

 that we perhaps do not realize what a great help it is to us. 

 Window glass is made of pure sand and compounds of the 

 two metals, calcium and sodium, lime and soda being com- 

 monly used. These are heated together at a very high 

 temperature and form liquid glass. As it cools, it becomes 

 pasty and can be worked into various shapes. 



