INSTRUMENTS FOR SOWING CULTURES 



B 



B. Platinum wires. 



Platinum is to be preferred to all other metallic wires because it does not 

 oxidize after being heated to redness. The wire must be suitably mounted 

 in a glass or metal handle, since on account of its high conductivity it cannot 

 otherwise be held in the fingers. 



A platinum wire (German, ose) so mounted meets all requirements. It is 

 convenient to have three sizes of wire, stout, 

 medium and fine : each will serve a special 

 purpose (fig. 55). The fine wire is the most 

 generally useful, because it cools more quickly 

 than the stouter wires, and this is an important 

 consideration in the successful sowing of cul- 

 tures. At the same time it has very little 

 rigidity and is easily bent, so that it cannot be 

 used for instance, to sow cultures which adhere 

 firmly to solid media, nor for sowing a rough- 

 surfaced medium such as potato. 



In the laboratory there should always be at 

 hand : 



A fine straight wire for sowing stab cultures 

 (fig. 55, A). 



A stout wire whose point is flattened in the 

 form of a spatula (fig. 55, B). 



A medium-sized wire, which can be bent to 

 any desired angle near its end (fig. 55, C). 



A fine wire bent into a loop at the end for 

 picking up a drop of fluid (fig. 55, D). 



Method of mounting platinum wires. 1. Take 

 a glass rod 5-7 mm. in diameter, and divide it into lengths of 20-25 cm. 

 by making a mark with a file and then breaking the rod at this mark between 

 the thumbs. 



2. Cut the requisite number of lengths of platinum wire with a pair of 

 strong scissors, making each 5-7 cm. long. 



3. Take one of the pieces of glass rod in the left hand, soften one end in 

 the blow-pipe, rotating it between the fingers meanwhile. With forceps in 

 the right hand pick up one of the pieces of platinum wire, holding it about 

 15 mm. from one end, and heat this end to a white heat in the flame. 



4. When the heated end of the glass rod is softened, push the hot end of 

 the platinum wire into it, so that a centimetre or more is embedded in the 

 rod. Heat for a few moments, [pull out slightly] and then allow to cool. 



5. Round off the rough edge of the other end of the glass rod in the flame. 



6. Then with a pair of dissecting forceps bend the projecting end of the 

 wire into a loop or at a right angle, or flatten it with a hammer, as the case 

 may be. 



Technique. 1. Hold the glass rod by its upper one-third, and pass the 

 other end to which the platinum wire is fused rapidly through the Bunsen 

 to destroy any organisms which may be present on the surface. 



This sterilization of the glass rod should be done rapidly, because the smooth 

 surface of the glass is quickly sterilized and moreover does not come into immediate 

 contact with the culture : if the glass be overheated there is a risk that it may 

 crack at the point where the wire is fused into it. 



2. Heat the wire to redness, and on taking it out of the flame let it cool 

 for a few seconds in the air. 



FIG. 55. Platinum wires. 



