240 POPULAR SCIENCE MONTHLY. 



the soil. The majority to be met with in air are derived from the 

 soil. Their number lessens when the surface soil is moist, and it in- 

 creases as the surface soil dries. In a dry season the number of air 

 organisms will tend to increase. 



Town air contains more bacteria than country air, whilst they 

 become few and tend to disappear at high levels and on the sea. A 

 shower of rain purifies the air greatly of bacteria. The organisms 

 being, as I stated, mainly derived from the surface of the ground, 

 their number mainly depends on the physical condition of the soil, 

 and this depends on the weather. Bacteria cannot pass independ- 

 ently to the air; they are forcibly transferred to it with dust from 

 various surfaces. The relative bacterial purity of the atmosphere is 

 mainly, therefore, a question of dust. Even when found floating about 

 in the air the bacteria are to be met with in much greater number 

 in the dust that settles on exposed surfaces, e. g., floors, car- 

 pets, clothes and furniture. Through a process of sedimentation 

 the lower layers of the air become richer in dust and bacteria, and 

 any disturbance of dust will increase the number of bacteria in the 

 air. 



The simple act of breathing does not disseminate disease germs 

 from a patient; it requires an act of coughing to carry them into the 

 air with minute particles of moisture. From the earliest times great 

 weight has been laid upon the danger of infection through air-borne 

 contagia, and with the introduction of antiseptic surgery the en- 

 deavor was made to lessen this danger as much as possible by means 

 of the carbolic spray, etc. In the same connection numerous 

 bacteriological examinations of air have been made, with the view of 

 arriving at results of hygienic value. The average number of micro-' 

 organisms present in the air is 500-1000 per 1000 liters; of this 

 number only 100-200 are bacteria, and they are almost entirely harm- 

 less forms. The organisms of suppuration have been detected in 

 the air, and the tubercle bacillus in the dust adhering to the walls of 

 rooms. Investigation has not, however, proved air to be one of the 

 important channels of infection. The bactericidal action of sunlight, 

 desiccation and the diluting action of the atmosphere on noxious 

 substances will always greatly lessen the risk of direct aerial infec- 

 tion. 



The physical agents that promote the passage of bacteria into the 

 air are inimical to their vitality. Thus, the majority pass into the 

 air not from moist but from dry surfaces, and the preliminary drying 

 is injurious to a large number of bacteria. It follows that if the air 

 is rendered dust-free, it is practically deprived of all the organisms 

 it may contain. As regards enclosed spaces, the stilling of dust and 

 more especially the disinfection of surfaces liable to breed dust or 



