PRINCIPLES OF VENTILATION 323 



foot. But the air admitted to the room contains but 1.3 

 grams; it therefore has a relative humidity of but 16.3 per cent. 

 This is less than one-third of the relative humidity of outdoor 

 air in the very dryest portion of the United States at this time 

 of year (see Fig. 221). The pupils in such a schoolroom are 

 often living in an atmosphere dryer than outdoor air at the 

 center of the Sahara Desert, or any inhabitable portion of the 

 earth's surface. 



Such air is greedy for water vapor and robs every object 

 in the room of all available moisture. As a consequence, the 

 floor-cracks open, the furniture begins to creak, every joint in 

 woodwork and furniture opens, even pianos made of the best 

 kiln-dried wood show the effects of the drought, leather backs 

 of books become dry and sometimes crack, and house plants 

 begin a struggle for life itself* 



355. Some Evil Effects of Such Dry Air. Is such dry air 

 beneficial to the human system? Physicians say it is not. 

 They tell us that such excessively dry air causes rapid evapora- 

 tion from the nasal passages, and from the throat and bron- 

 chial tubes, and thus keeps the mucous membrane in a constant 

 state of irritation; the mucous membrane thus irritated 

 becomes swollen and spongy and affords an easy lodging place 

 for disease germs. 



Even physicians who do not object to very dry air, or even 

 see benefits to be derived from living in a dry climate con- 

 stantly, are among those who object most seriously to this 

 desert-like air in our homes and schoolrooms in northern 

 latitudes. They object most seriously to the change we 

 necessarily encounter when we are obliged to step from an 

 indoor atmosphere heated to 70 with a humidity of 20 per 

 cent, into an outdoor atmosphere at 20 with a humidity of 

 80 per cent. 



356. Dry Air Requires High Temperature. One has but to 

 consider the effect of dry air upon the wet- and the dry-bulb 

 thermometers to realize that a high temperature is necessary in 

 order that we may be comfortable in a room having such low 



