Io BACTERIOLOGICAL DIAGNOSIS. 
in the same way and are sometimes requisite for the 
transmission of fluids to a public laboratory for an 
examination which the practitioner is unable to carry 
out at home. They may also be sterilised by boiling. 
Test-tubes are treated in the same way as flasks. Petri 
dishes are wrapped round with tissue-paper or filter 
paper before being sterilised. 
Before removing glass apparatus from the steriliser 
remember to let the temperature fall gradually, or the 
vessels may crack. In the case ofa proper steriliser or 
of a biscuit tin heated by a burner the gas is turned out 
and the whole apparatus allowed to cool before the door 
is opened. In the case of a kitchen oven the best plan 
is to let the fire go out, or to open the door very 
gradually. 
Cotton-wool is sterilised by being spread out in thin 
layers on the shelves of the apparatus, and the heat 
is continued until the outside is singed. 
Metal instruments (knives, scissors, &c.) may be steril- 
ised in the same way and at the same time. They 
should be wrapped loosely in cotton-wool, and should 
not be removed from their wrapping until the moment 
at which they are to be used. 
Steam is chiefly used for the sterilisation of culture 
media before use, and for the destruction of cultures 
when they are done with. The latter purpose, however, 
is accomplished more speedily and safely by the ad- 
dition of a few drops of commercial formalin to each 
tube. 
The proper steam steriliser consists of a metal 
cylinder with a perforated diaphragm six or eight 
inches from the bottom. It is enclosed in a thick layer 
of felt or other non-conductor of heat, and is provided 
with a lid. The space.between the bottom and the 
