EXAMINATION OF AIR, SOIL AND WATER, 151 
In examining in-doors air, what must be taken into account? 
The length of time the air of the room has been quiet, and the 
length of time that has elapsed since the room was swept or dust 
raised in any way. 
What average results have been obtained in different places by 
examinations of this kind ? 
The number of bacteria present in ten litres (about six hundred 
cubic inches) of air vary widely according to the conditions present. 
Examinations have shown the average to be as low as ten and as many 
as three hundred and seventy-six bacteria present in ten litres of air at 
different times and places. 
In what way are bacteriological examinations of soil made ? 
The sample of soil may be spread over a plate of nutrient gelatin, 
or may be mixed directly with the culture medium. It is extremely 
difficult to get soil from the deeper strata that is uncontaminated with 
material from strata lying above. However, it has been determined 
that superficial soil is very rich in bacteria, and that the deeper we go 
the less abundant they are. In soil below the level of the ground 
water, it is fair to presume that no bacteria are present, as undisturbed 
soil forms a very efficient filter. 
How are bacteriological examinations of water conducted ? 
The water to be examined is put into sterilized flasks stopped with 
cotton wool and covered with a rubber cap. It must be examined as 
soon as possible after it is procured. 
The water is carefully shaken before examination to distribute the 
organisms present as equally as possible. 
A known portion of the water is taken with a sterilized pipette 
and mixed with a sterilized fluid nutrient gelatin. From this a plate 
or dish culture is made, and calculations made accordingly. 
Is the actual number of bacteria present in a given quantity of 
water of any real significance’? 
No. A cubic centimetre of water may contain several thousand 
bacteria and all may be harmless. On the other hand, the water ex- 
amined may be almost free from bacteria and still be very dangerous to 
life: for the reason that one or more individual organisms present may 
be pathogenic, e. g., typhoid fever bacilli or cholera vibrios. 
As a rule, which contains the greatest number of bacteria, river 
water or ground water, and why? 
River water: for the reason that usually the soil covering the under- 
