136 TRANSACTIONS OF THE PLYMOUTH INSTITUTION. 
and not in another. Sometimes liquid media are used, as chicken 
broth, beef tea, &c. ; and at others solid media, such as potatoes 
cut open with a sterilised knife, or nutrient jelly, made frequently 
of beef juice, and salt and peptone — i.e. albumen, acted on by 
the gastric juice — thickened with agar-agar (Ceylon moss). All 
the glass vessels, test tubes, &c, are carefully cleansed and 
sterilised by heat, lest some germ other than that which it is 
desired to cultivate should be introduced into the cultivating 
medium. After being cleansed and stoppered with a plug of 
cotton wool, they are placed in the hot-air chamber at 150° C. 
for an hour, which destroys any germs in the vessels or cotton 
wool. As the jelly may be contaminated in being poured into 
the test tubes, this process must be repeated after the jelly 
has been placed in the tubes. For this purpose they are placed 
in a steaming apparatus for fifteen minutes, and this is repeated 
for three or four successive days. To inoculate with the germs 
which it is wished to cultivate, a needle of platinum wire is used, 
fixed at the end of a glass rod. Platinum wire does not retain 
heat, so it can be sterilised by being passed through a flame 
quickly and then used at once. The cotton plug is removed, and 
the wire, charged with the substance to be examined, is traced on 
the surface of the cultivating medium, and so the germ is sown, 
so to speak. 
By the way in which the germ grows the nature of it is 
recognisable; some forms spreading downwards, and others 
laterally. A form called Proteus vulgaris, which is sometimes 
found in the matter of an abscess, grows rapidly in nutrient jelly, 
liquefying it. If grown in meat infusion, small oval bodies will 
predominate ; but if in nutrient jelly, on a sterilised glass plate, 
if examined within a few hours of inoculation, a different 
appearance will be found. Growth appears along the track of 
the needle, and quickly spreads over the jelly. In a few hours 
small transparent colonies are seen on the surface, and these, 
when examined microscopically, are found to consist of large 
bacilli, or rod-shaped bodies. As these colonies get older they 
become denser in the centre, and the bacilli shorter ; while at the 
circumference very long threads are seen projecting outwards into 
the gelatine. After a time it will be found that the line of the 
needle is quite liquid, and contains mainly the short bacilli ; 
but from the margins of the track, and speading over the still 
