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the surrounding atmosphere and of man's relation to it. Infectious diseases, of course, form an important part in the consideration of these facts. 
As a very good example of the kind of questions which are considered in connection with those that are purely chemical, is perhaps the following:
In the chapter, treating of the currents of air in the soils of different composition and densities and the conditions of temperature and moisture upon which their movements depend, he approaches the question of why malarial infection has been observed to take place more easily at night than during the day. According to physical investigations, air currents from soil, and, carrying with it germs, into the surrounding atmosphere, ought to attain their maximum of rapidity at the time when the difference in temperature between these two media is greatest,