BACILLUS AEROGENES CAPSULATUS.       465
the stain. The specimen is then examined in the color-
ing-solution, after soaking up the excess with filter paper,
the thin layer of coloring fluid not interfering with a clear
view of the bacteria and their capsules. After mounting
in Canada balsam the capsules are not nearly so distinct.
The width of the capsule varies from one-half to twice
the thickness of the bacillus. Its outer margin is stained,
leaving a clear zone immediately around the bacillus.
It was at first thought that the bacillus produced no
spores, but Dunham1 found that spores were produced
upon blood-serum, and especially upon Loffler's blood-
serum bouillon mixture. The spores resist desiccation
and exposure to the air for ten months. They stain
readily in hot solutions of fuchsin in anilin water, and
are not decolorized by a moderate exposure to the action
of 3 per cent, solution of hydrochloric acid in absolute
alcohol. They are oval, and are usually situated near
the middle of the bacillus, which is distended because of
the large size of the spore and bulges at the sides.
The bacillus is anaerobic. It grows upon all culture-
media, both at the room-temperature and at the tempera-
ture of incubation, best at the latter. The bacillus grows
in ordinary neutral or alkaline gelatin, but better in gela-
tin containing glucose, in which the characteristic gas-
production is marked. Soft gelatin, made with 5 instead
of 10 per cent, of the crude gelatin, is said to be better
than the ordinary medium.
There is no distinct liquefaction, but in 5 per cent,
gelatin there is sometimes a softening that can be best
demonstrated by tilting the tube and observing that the
gas-bubbles change their position, as well as by noticing
that the growth tends to sediment.
In making agar-agar cultures careful anaerobic precau-
tions must be observed. The tubes should contain con-
siderable of the medium, which should be boiled and
freshly solidified before using. The implantation should
be deeply made with a long wire. The growth takes
1 Bull, of the Johns Hopkins Hospital, April, 1897, p. 68.
30