128 STERILIZATION AND DISINFECTION 
and unchanged. Body extracts and scra may be thus treated to render 
them sterile. 
Filtration through Liquids. The action is entuely mechanical 
unless some strong liquid such as sodium hydroxide or sulphuric acid 
are used. In these cases the bacterial protein is decomposed by the 
chemical. Rettger applied this method to the bacterial examination 
of air. 
Filtration through Solids. The same principle is involved here 
that is involved in the use of the cotton plug in test tubes and culture 
flasks, Different substances have been used but most attention has 
Fia. 41.—Diatomaccous Filters for the Sterilization of Bacterial Extracts and Body 
Fluids. 
been given to cotton, glass, wooland sand. Sand is used in the standard 
method for the bacterial examination of air and also in the purification 
of water. Stone or porcelain filters are the best substances for labora- 
tory use. They may be secured in many sizes and are easily sterilized 
and cleaned. 
Dialysis. In this medium the semi-permeable membrane is used 
as the filtering medium. Collodion sacs have had quite a little use 
in bacterial work. 
Licut 
Although light does exert an antiseptic action on bacteria and 
may even totally destroy them after a long period, it is scarcely used 
