BACTERIOLOGICAL TECHNIQUE. 931 
to new media, and these grow, we have what we call a 
pure culture of that variety. When these are trans- 
ferred to the solid media we call the growth which 
takes place from smearing the bacteria over the surface, 
a surface or smear culture, and that formed in the 
depth of the media by plunging the needle carrying 
the bacteria into it, a stab culture. (Fig. 31.) 
In transferring bacteria from one tube to another we 
slant the tubes so that no dust may fall within and 
contaminate with other bacteria the special variety we 
wish to transplant. The greatest care must be taken 
that the sterilized platinum needle used to transfer the 
bacteria is not infected by touching any non-sterile 
matter. Even with our utmost care bacteria will from 
time to time pass from the air or edges of our tubes 
into the culture media, and thus possibility of contami- 
nation must always be kept in mind. When it occurs 
upon solid media we, as a rule, easily detect it, for we 
notice the growth at some point of bacteria of different 
colony characteristics; but in fluid media, on account of 
the complete mingling of the bacteria, we are not so apt 
to notice the additional growth. 
Incubators. In order to have a constant and proper 
temperature for the growth of bacteria, forms of appa- 
ratus called incubators have been devised. These (Fig. 
32) consist in their simplest form of an inner air cham- 
ber surrounded by a double copper wall containing water. 
The apparatus externally is lined with asbestos, to pre- 
vent radiation. It is supplied with doors and with 
openings for thermometers and a thermo-regulator. The 
thermo-regulators are of various kinds; those in most 
use depend upon the expansion or contraction of the _ 
fluid in a bulb (A, Fig. 33), which rests within the 
