230 APPLIED PHYSIOLOGY 



break the air current and allow the dust to settle. In some, the air is 

 passed through a layer of cotton. Cold air contains less moisture than 

 warm air, and unless the air is given more moisture before it is sent to 

 the rooms, it will be very dry. So a pan of water should always be 

 kept inside the air box of a furnace. 



402. Schoolroom ventilation. Children are especially 

 susceptible to unhealthful surroundings, and the air of 

 a schoolroom, in which they spend the greater part of the 

 day, should be kept pure. Pure air means clearer brains 

 and better lessons, and may determine whether or not a 

 child shall gain a sufficient knowledge to assure his suc- 

 cess in life. In every half day of school it is well to allow 

 a short recess in which windows and doors can be thrown 

 wide open and the pupils sent out to get deep breaths of 

 oxygen during play. 



The upper sashes of all the windows on the side of the 

 schoolroom away from the wind can be kept open a space 

 so as to produce a gentle outward current of foul air. 



If the upper sashes cannot be lowered, the lower one 

 can be raised and a board inserted under it so that the 

 only opening left is between the two sashes. 



If registers or special means of ventilation are provided, 

 they should be watched and regulated according to the 

 needs of the air. 



403. Purification of the atmosphere. Although it is con- 

 tinually receiving impurities, the atmosphere as a whole never becomes 

 foul, for the process of purification never ceases. First, the wind scat- 

 ters the impurities to a height miles above our heads and over the seas 

 to arctic and uninhabited regions, and thus dilutes the impurities. 

 Second, rain washes out dust and germs and soot, and foul gases, and 

 carries them into the earth. Third, sunlight destroys living germs 

 floating in the air, and dries up stagnant sources of impurities. Fourth, 

 plants, both on land and in the sea, absorb carbonic acid gas and 

 restore the oxygen to the air. By these means the composition of the 

 air is kept always the same. 



