the only means of discovering the nature of the organisms, for characteristics deduced from their shape are of no significance. Most frequently they can he distinguished only by the fermentations they produce. Numerous experiments based on this principle will be required for the exact determination of the bacteria in the atmosphere. In the present condition of the science, we have to limit ourselves to the general statistics of the Micrococci, Vibrios, Bacteria, Bacilli, and Cladothrices that live in the air, without undertaking to classify in any precise way all the beings comprehended under each of these denominations.
The observations conducted at the Observatory of Montsouris show that there are on the average eighty bacteria in a cubic metre of air. The highest number was observed in the fall, the lowest in the winter. There were found fifty bacteria in December and January, only thirty-three in February, one hundred and five in May, fifty in June, and one hundred and seventy in October. The diagrams of daily observations
Fig. 8.—Influence of the Direction of the Wind on the Number of Aërial Microbes collected at Montsouris.
show that the number of spores of these alga? increases with the temperature. Inversely to what takes place in the case of the molds, the number of the schizophytes, small in rainy weather, rises when all the moisture has disappeared from the surface of the soil. The counter-action of moisture is stronger than the direct action of temperature; and this fact accounts for the rarity of the bacteria after the great rains of February, April, and June. Still a long period of dry weather does not appear to be favorable to the development of the plants.
