BACTERIOLOGICAL TECHNIQUE. 231 



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 

 ther mo-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 



