MICROSCOPIC LIFE IN THE AIR. 



3 



compare different periods of the same year, we shall not find the vari- 

 ations so regular. Sometimes the number of germs diminishes while 

 the heat is increasing. In that case the effect of temperature is 

 masked by the preponderance of another factor the hygrometric 

 condition of the air. This fact is explained by remembering that 



Fig. 3. Spores of Alg-e and Molds in the Atmosphere. 



the development of molds is dependent upon both heat and moisture. 

 The effect of moisture, however, varies according to the season, and 

 with changes in temperature. Dry weather diminishes the number of 

 germs in summer, and increases it in winter, while moist weather oper- 

 ates in an inverse manner. 



Storms in the pleasant season are followed by a growth of cryp- 

 togamic vegetation, and purify the atmosphere for only a very short 

 time. Fifteen or eighteen hours after a rain, says M. Miquel, " the 

 spores appear to be five or ten times as numerous as before. On the 

 other hand, mineral dusts and several kinds of microbes continue to 

 be rare till the moisture which has caused them to adhere to the blades 

 of grass and the moist soil of the surface has dried away." 



These investigations, while they are profitable in a purely scientific 

 aspect, are also destined to be of service in agriculture and hygiene. 



