170 PHOTO-MICROGRAPHS. 
the year, a yellow streak is often left upon the 
surface of the moist earth along the margins of 
the gutters. Upon microscopical examination, 
this will be found to be made up of these bi- 
lobate pollen grains from the pine which have 
been washed from the atmosphere by the falling 
rain. There are, however, no pine forests in the 
immediate vicinity, and the pollen is evidently 
transported many miles by the winds. This fact 
illustrates the bountiful supply of these repro- 
ductive elements provided by nature, and also 
furnishes an important hint to sanitarians. If 
pollen grains can be carried such long distances, 
we may be sure that the minute organisms and 
their germs (spores) which find their way into 
the atmosphere from all sorts of foul hot- 
beds may also be transported in the same way, 
and that they too are washed out by the falling 
rain. So that rain-water, commonly considered 
the purest water to be had for drinking, is in 
reality “ the sewage of the atmosphere,” as it has 
been aptly called by a distinguished surgeon of 
the army. The sanitary lessons to be drawn 
from these facts are evident. Preserve the at- 
mosphere from contamination by destroying filth 
—organic matter undergoing decomposition — 
by covering or draining exposed mud-flats, ete., 
and reject the rain-water which first falls when 
there is reason to believe that the atmosphere 
is contaminated with these germs of putrefac- 
tion or of disease— during the summer months, 
especially in cities, and during the prevalence of 
an epidemic. 
