NITROGEN. THE ATMOSPHERE 289 



below 0, the fog is composed of solid, and not of liquid particles. 

 The hexagonal crystalline structures of ice which are deposited 

 form snow. 



On the other hand, when the weather becomes warm, evapo- 

 ration goes on rapidly, especially in the neighborhood of seas, 

 lakes, or moist country, and the proportion of water vapor in 

 the air may be considerably increased. 



Humidity. The moisture is usually defined in terms of 

 relative humidity, the standard being the quantity required to 

 saturate the air at the existing temperature. A space filled with 

 air can take up aqueous vapor only until the partial pressure of 

 water vapor becomes equal to the vapor pressure of water (p. 61) 

 at the same temperature. The humidity is then said to be 100 

 per cent. If the partial pressure actually reached is only half as 

 great as the vapor pressure of water at the same temperature, the 

 humidity is 50 per cent. The average humidity may be placed 

 very roughly at about 66 per cent. 



At 18 (64.4 F.), the vapor pressure of water is 15.4 mm. 

 (Appendix IV). If the total pressure of the atmosphere were 

 760 mm., then the air would be saturated with moisture at 18, 



15 4 

 and have a humidity of 100 per cent, when -=^=- or about 2 per 



cent of it by volume was water vapor. Upon cooling to 0, at 

 which temperature the vapor pressure of water is 4.6 mm., this 



4 6 

 air would retain only ^, or about 0.6 per cent of moisture. At 



18 there would be 16.3 grams of water in a cubic meter of air 

 and at only 4.9 grams. The difference, 11.4 grams (11.4 

 c.c.), would be precipitated as fog or rain from each cubic meter. 



Test for Moisture in Air. The presence of moisture in air 

 may be shown by placing any deliquescent (p. 118) salt, such as 

 calcium chloride, in an open vessel. The quantity can be meas- 

 ured by driving a known volume of air slowly through a weighed 



