compare different periods of the same year, we shall not find the variations 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 Algæ 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 operates in an inverse manner.
Storms in the pleasant season are followed by a growth of cryptogamic 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.
