44 COMMON PHENOMENA DUE TO HEAT 



Proper ventilation eliminates much of the physical danger 

 of social events and fresh, dry air should be constantly ad- 

 mitted to crowded rooms in order to replace the air satu- 

 rated by the breath and perspiration of the occupants. 



Weather forecasts. When the air is near the saturation 

 point, the weather is oppressive and is said to be very humid. 

 For comfort and health, the air should be about two thirds 

 saturated. The presence of some water vapor in the air is ab- 

 solutely necessary to animal and plant life. In desert regions 

 where vapor is scarce the air is so dry that throat trouble ac- 

 companied by a disagreeable tickling is prevalent; fallen leaves 

 become so dry that they crumble to dust; plants lose their 

 freshness and beauty. 



The likelihood of rain or frost is often determined by tem- 

 perature and humidity. If the air is near saturation and the 

 temperature is falling, it is safe to predict bad weather, be- 

 cause the fall of temperature will probably cause rapid con- 

 densation, and hence rain. If, however, the air is not near 

 the saturation point, a fall in temperature will not necessarily 

 produce bad weather. 



The measurement of humidity is of far wider importance 

 than the mere forecasting of local weather conditions. The 

 close relation between humidity and health has led many 

 institutions, such as hospitals, schools, and factories, to regu- 

 late the humidity of the atmosphere as carefully as they do 

 the temperature. Too great humidity is enervating, and not 

 conducive to either mental or physical exertion ; on the other 

 hand, too dry air is equally harmful. In summer the humidity 

 conditions cannot be well regulated, but in winter, when houses 

 are artificially heated, the humidity of a room can be increased 

 by placing pans of water near the registers or on radiators. 



Heat needed to melt substances. If a spoon is placed 

 in a vessel of hot water for a few seconds and then removed, 



