172 BACTERIOLOGY. 



from which transplantations can be readily made. Colon- 

 ies of bacteria which would escape detection when growing 1 

 on the surface of a potato, because they are invisible, are 

 easily seen in the solid, transparent layer of g'elatin. Pure 

 cultures, of the different kinds of bacteria, are thus ob- 

 tained. 



Method. First, sterilize six glass plates (13 X 10 cm.) 

 by placing 1 them in an iron box, and heating 1 this in the dry- 

 heat sterilizer, at a temperature of 150-175, for >^-^ 

 of an hour. A temperature of 170 maintained for 15 min- 

 utes, will be sufficient (see p. 160). Remove the box, and let 

 it cool in the room at the ordinary temperature. 



FIG. 28. Apparatus used in cultivation, a Glass benches and plates; b Moist 

 chamber for plates; c Petri dish; d Esmarch dish; e Platinum wires, straight and 

 with loop, fused into the ends of glass rods. 



In the meantime, prepare the necessary platinum wires. These 

 should be about 5 cm. (2 in.) in length, and in thickness should corre- 

 spond to about gauge number 22 (0.5 mm.). The end of a glass rod, 

 about 18 cm (7 in.) long and 6 mm. (% in.) wide, is heated in a Bunsen 

 flame, or better still, in a blast lump, till it has become quite soft. 

 The platinum wire is then pushed into the soft end. If a depression 

 occurs around the wire, where it enters the glass, this can be easily 

 remedied by gently pulling on the wire, while the glass is still soft. 

 A glass tube should never be used for mounting platinum wires. 



In order to prevent sudden, uneven cooling, it is well to partially 

 anneal the glass, by allowing a small luminous flame to deposit a 

 layer of lamp-black on the hot end. This is then wiped off when the 

 glass is perfectly cool. It is well to have on hand three mounted, 

 platinum wires. The wire should be kept perfectly straight, not 

 crooked. When plating or doing similar work, the end is bent into a 

 small loop which has a clear diameter of about 2 mm. (Fig. 28 e). 



