TESTING AIR, WATER, AND EARTH, FOR IMPURITIES. 319 
Pe StinGg Ak WATER ANDIEAR EE PO 
IMPURITIES. 
N his paper on the above subject, read at the Berlin Medical 
Congress, Dr. Koch remarked that at one period the three 
‘elements ” named were considered as capable of transmitting in- 
fection, without any- exact theories being perfected as to their 
physical and chemical relations. With respect to air, he referred 
at some length to the process of filtration through gun cotton, and 
to the system of conducting (by means of an exhausting apparatus 
and supplementary appliances) a certain quantity of air to a plate 
of glass which was afterwards subjected to microscopic examination. 
Another form of test was that of bringing a current of air through 
so-called ‘‘ nourishing fluids ” in which the spores were developed. 
The subject of the various forms of “nourishing grounds” formed 
an important portion of Dr. Koch’s paper, and detailed reference 
was made by him to the method adopted by Hesse for defining the 
exact quantity of air from which the spores originated, the develop- 
ment of which forms an important part of Dr. Koch’s experimental 
method. A glass tube, about r2in. long and 23éin. wide, is closed 
at each end with india rubber coverings, in one of which a glass 
pipe is inserted, while in the middle of the other is an opening 
about three-eighths of an inch-in diameter. Gelatine is placed along 
the bottom of the tube, which is in a horizontal position. The 
smaller pipe is then placed in connection with an exhausting 
apparatus, and a given quantity of air is forced through, the bacteria 
and bacteria-spores falling on the gelatine, which is described as 
being covered most thickly with them in the vicinity of the place 
where the air enters, while the opposite end of the tube is usually 
free from them. ‘The quantity of air usually tested is said to be 
about twelve to fifteen litres. It is remarked that this process is a 
decided improvement upon the older method of assuming the 
quantity of carbonic acid in the air as being a measure of its im- 
purity. According to the newer theory, air may be comparatively 
free from carbonic acid and still contain many noxious germs, 
while, on the other hand, air containing an excess of that acid may 
have but few of these germs. They are, however, found in 
relatively abundant quantity when there is dust in the air. The 
tests for water and earth are described as being carried out on the 
same principle, but with the necessary modifications. In the 
former case the gelatine is rendered fluid by heat, and the required 
quantity is introduced into a test-tube of moderate size, a drop 
of the water to be examined being added, and the opening closed 
with wadding. After two or three days the bacteria can be dis- 
