20 SCIENCE OF HOME AND COMMUNITY 



But experiments have shown that neither of these is the 

 cause, since under ordinary conditions practically all air 

 contains enough oxygen for breathing, and practically no 

 air contains enough carbon dioxid to be injurious. Another 

 explanation was that there were tiny organic particles given 

 off in breathing which produced the ill effects ; but experi- 

 ments seem to show that these are not the cause ; but dis- 

 agreeable odors, due to particles given off from the body, 

 from decaying teeth, and from clothing, render the air very 

 unpleasant and may at times produce injurious results. 



Physical causes of ill effects. The cause for the ill effects 

 of poor ventilation is thus seen not to lie in the lack of chemical 

 purity of the air, but must be sought in its physical conditions. 

 Ventilation is not merely a matter of breathing alone, but 

 also a matter of the reaction of the skin to the air with which 

 it is in constant contact. 



Recent experiments seem to show that in seeking the 

 essentials of ventilation, we should consider the effect of the 

 air on our skin rather than the effect of the air we breathe 

 on our lungs. 



How the body controls its temperature. In order to under- 

 stand what is needed for ventilation, one needs to know some- 

 thing of the part that the skin plays in the control of the heat 

 of the body. Heat is produced in the body by the oxidation 

 of food. The temperature of the body is kept constant, 

 about 98 degrees, and the body is provided with means by 

 which this temperature is automatically kept uniform . There 

 are two methods by which the body loses heat, first by send- 

 ing large quantities of blood to the surface of the skin, where 

 it is cooled ; and second, by the evaporation of water from 

 the pores of the skin, in what is called perspiration. If the 

 body is too warm, the arteries in the skin open wide and 

 thus allow large amounts of blood to come to the skin, where 

 cooling takes place; and the sweat glands increase their 

 activity in evaporating water. On the other hand, if the 



