

GERMS IN AIR AND WATER 41 
in their abundance as mountain elevations are 
ascended, or over the sea as we recede from the 
land. ‘ Pathogenic bacteria, on the other hand, are 
only occasionally present in the air. The practical 
results obtained from the examination of air for 
pathogenic bacteria have been slight. We know 
that at times they must be in the air, but unless we 
purposely increase their numbers they are so few in 
the comparatively small amount of air which it is 
practicable to examine that we rarely find them. 
Examination of dust, however, in hospital wards and 
sick-rooms, has revealed tubercle bacilli and other 
pathogenic bacteria.” ! 
Much the same kind of thing has to be said con- 
cerning waters of different kinds; they all contain 
various kinds of Bacteria: even the purest waters 
are not free from them, and ordinary tap waters 
contain them in abundance. River water becomes 
contaminated by washings from the soil after rains, 
as well as from the contents of drains and sewers. 
The results of such contamination are fortunately 
rendered less disastrous by the fact that a natural 
process of purification occurs tending to diminish the 
eve) (ius Park says. (doc. ¢7.p. 449): “ That 
river water which has been fouled by sewage will, 
in the course of a few miles, through the dilution of 
additional supplies, through sedimentation, and 
through oxidation, become greatly purified is an 
indisputable fact.” 
But in reference to experiments on the question 
of the possible occurrence of Archebiosis we need 
1 Park’s “ Pathogenic Micro-organisms,” 2nd ed., 1906, p. 448. 
