54 A MANUAL OF BACTERIOLOGY 



animals, platinum needles or injection syringes (Fig. 

 20) are used; but in every case these instruments 

 must be thoroughly sterilised before use. As Petri's 

 and Koch's syringes cannot be heated without de- 

 struction, they are sterilised by being immersed in 

 a solution of mercuric chloride or mercuric iodide ; 

 and after this these syringes should be washed with 

 sterilised hot water. 



In addition to the above, ' glass needles are espe- 

 cially useful when anaerobic microbes are being 

 dealt with, as the smooth surface of the glass does 

 not allow of oxygen (air) being carried down with 

 it along the track, which closes up as soon as the 

 needle is withdrawn.' 



We now proceed to describe the solid media 

 beginning with nutrient gelatine. This is made 

 according to the process already described for the 

 preparation of bouillon, except that after the filtra- 

 tion of the neutral or slightly alkaline fluid, 100 

 grammes of the best gelatine, 1 10 grammes of pep- 

 tone (albumin), and 5 grammes of common salt are 

 added. The gelatine is allowed to soften and 

 dissolve gradually by gently heating the mixture in 

 a water bath. The nutrient gelatine is then steril- 

 ised as usual, and filtered into tubes or flasks where 

 it solidifies. The tubes and flasks being filled with 

 the nutrient gelatine must be sterilised in a steam 

 steriliser for a quarter of an hour on three successive 

 days. If these tubes show no signs of turbidity 

 after about a week's incubation, they may be con- 

 sidered sterile. 



1 Coignet's gold label gelatine is the best for this purpose. 



