NUTRIENT MEDIA 147 



for the solidification of those media it is intended to 

 use in the cultivation of bacteria at the room tem- 

 perature or in the "cold" incubator. In the percent- 

 ages usually employed, gelatine media become fluid at 

 25 C.; higher percentages remain solid at somewhat 

 higher temperatures, but the difficulty of filtering 

 strong solutions of gelatine militates against their gen- 

 eral use. 



Media, on the other hand which have been solidified 

 by the addition of agar, only become liquid when ex- 

 posed to 90 C. for about ten minutes, and again solidify 

 when the temperature falls to 40 C. 



When it becomes necessary to render these media 

 fluid, heat is applied, upon the withdrawal of which they 

 again assume their solid condition. Such media should 

 be referred to as liquefiable media; in point of fact, 

 however, they are usually grouped together with the 

 solid media. 



NOTE. It must here be stated that the designation 10 per 

 cent, gelatine or 2 per cent, agar refers only to the quantity of 

 those substances actually added in the process of manufacture, 

 and not to the percentage of gelatine or agar, as the case may be, 

 present in the finished medium; the explanation being that the 

 commercial products employed contain a large proportion of 

 insoluble material which is separated off by filtration during the 

 preparation of the liquefiable media. 



Other media, again e. g., potato, coagulated blood- 

 serum, etc. cannot be again liquefied by physical 

 means, and these are spoken of as solid media. 



The following pages detail the method of preparing 

 the various nutrient media, in ordinary use (see also 

 Chapter XI), those which are only occasionally required 

 for more highly specialised work are grouped together 

 in Chapter XII. It must be premised that scrupulous 

 cleanliness is to be observed with regard to all ap- 

 paratus, vessels, funnels, etc., employed in the prepara- 

 tion of media; although in the preliminary stages of 



