THE MICRO-ORGANISMS PRESENT IN THE ATMOSPHERE. 
129 
By bending tlie lead tubing the experimental tube (AB) is brought into a horizontal 
position, and in out-of-doOr experiments the open extremity (A) is turned away at an 
angle from the wind. A control-tube is attached by means of wire to the vertical 
support, so as to rest in a precisely similar position to the experimental tube. The 
long piece of lead tubing enables the operator to aspirate the air by means of the 
hand-pump without his movements disturbing the air in the vicinity of the experi¬ 
mental tube, whilst, by means of a mirror placed obliquely on the ground, he is able 
to watch the rise and fall of the mercury in the pressure-tube. After each upward 
stroke of the pump, the operator waits until the pressure is equalised before making 
the down-stroke, and by observing this precaution each stroke of the pump corre¬ 
sponds to the passage through the experimental tube of a definite volume of air, 
which is determined once and for all by means of a gas-meter. Thus, the number of 
strokes measures the volume of air aspirated, whilst the number of strokes performed 
per minute indicates the degree of perviousness which the plugs possess. In the case 
of the air-pump used in the experiments recorded both above and below, 8 strokes 
were equivalent to -^-th cubic foot, or 2’36 litres, 100 strokes corresponding to 
30 litres. 
The volume of air aspirated is varied, of course, according to the number of 
microbes supposed to be present, but with ordinary London air GO strokes of the 
pump, or 18 litres, were found to be convenient. 
In order to make what is practically a duplicate experiment, it is my general 
practice to alternate the aspiration of air through two different experimental tubes, 
taking 20 strokes through one, then 20 through the other, and so on ; between the 
aspirations the tubes are kept in their sterile case. When the desired volume of air 
has been aspirated through the tube or tubes the latter are at once replaced in the 
sterile case, and the further treatment of the plugs proceeded with as below described. 
3. Further Treatment of the Plugs : Flask Cultivation. —The ingenious modification 
of the method of plate cultivation recently introduced by my friend Dr. Esmarch, of 
Berlin (‘Zeitschr. f. Hygiene,’ 1886, p. 293), in which the gelatine, instead of being 
poured out upon a glass plate, is spread over the interior surface of the test-tube, and 
there congealed, appeared to offer particular advantages for the treatment of the air- 
plugs in question. I found, however, that test-tubes were not suitable for the 
purpose, as the gelatine could not be violently agitated with the plug so as to 
disintegrate the latter without causing an inconvenient amount of froth, which, on 
solidification, would seriously obscure the gelatine film. In order to obviate this, and 
to obtain a much larger surface of gelatine, I employ flasks, 300-400 c.c. # capacity, 
and containing about 12 c.c. of gelatine-peptone. These flasks, with their contents, 
are sterilised in the ordinary way by intermittent steaming for three days. 
The gelatine in these flasks having been melted at a temperature of 30° O., the 
* If a very large volume of air lias been aspirated, or if the air under examination is particularly 
rich in micro-organisms, flasks of greater capacity may be advantageously employed. 
MDCCCLXXXVII.— B. S 
