GLASS 221 



house has its power of holding water increased by warm- 

 ing. The actual amount of water is not changed; but the 

 amount that the air holds as compared with what it is 

 able to hold is decidedly smaller. Hence it takes too 

 much moisture from our bodies. A well-known authority 

 says that the moisture in the air of a room should be such 

 that dew and frost are deposited on the inside of windows 

 in cold weather. If steam or hot water are used for heat- 

 ing, a shallow dish of water should be kept in the room. 

 In the hot-air furnace a water pan is provided inside the 

 air box. This should be kept full. 



250. Light in the House. The problem of lighting 

 the house properly is a very serious one. Primitive 

 peoples, and even our own ancestors of a few generations 

 ago, could afford but little sunlight in their houses, free 

 as it is out of doors. This was because they had no cheap 

 transparent substance like our modern glass. They had 

 only oiled paper, or thin sheets of horn, isin-glass (the 

 mineral mica; cf. 285), etc. But the common people 

 could not afford, or get, much of these materials; so their 

 houses were dark. The problem of sunlight is so impor- 

 tant to the general health, as well as to sight, that the men 

 who made cheap glass possible must be ranked among 

 the greatest benefactors of the race. Darkness in the 

 house means dirt and filth, and where these exist disease 

 and pestilence are sure to get a foothold. 



251. Glass. Glass is made out of a mixture of sand, 

 limestone, and soda (or sodium sulphate instead of soda). 

 The mixture is melted in fire-clay pots about 4 feet high 



