26 BACTERIOLOGY 



one-tenth normal sodium hydrate and hydrochloric acid solu- 

 tions should be available; also, a 0.5 per cent solution of phenol- 

 phthalein in 50 per cent alcohol. Care should be taken to 

 prevent the absorption of carbon dioxide by the soda solution, 

 by arranging that all air which comes in contact with the latter, 

 either in the stock bottle or in the burette, shall first pass 

 through a strong solution of sodium or barium hydrate. The 

 arrangement of the apparatus is described in any work on 

 chemical analysis. The medium which has been previously 

 boiled for at least several minutes to expel carbon dioxide is 

 brought to the desired volume with water, and thoroughly mixed. 

 Media are commonly warm or hot when measured, hence it 

 must be remembered that true volumes cannot be thus, 

 obtained ; for instance, a litre measured at, say 80° C, would 

 be only 973 cc. if measured at 20° C, the temperature at which 

 litre flasks are calibrated. Since many media cannot be cooled 

 to 20° C. because of solidification, as in the case of agar or 

 gelatin, it is a better plan to determine measures of volume by 

 weight. For this, place a clean dry saucepan, in which the 

 medium is to be prepared, upon one side of a trip scale, and 

 counterbalance its weight exactly. The weight of a litre of 

 bouillon, gelatin, or agar, having been determined, once for all, 

 the necessary weights added to the weight of the pan will give the 

 amount which the pan and its contents must balance when 

 the volume is exactly one litre. A portion of the medium 

 brought to the exact volume is then taken and cooled to room 

 temperature (20° C), or to a point a few degrees above 

 solidification, and 10 cc. withdrawn, placed in a small beaker, 

 50 cc. of distilled water and i cc. of the phenolphthalein solution 



N' 

 added. If the medium is acid, the — NaOH solution is then 



10 



run in cautiously until a pale but decided pink color is obtained. 



The number of cubic centimetres of the solution used, multiplied 



by ten, will give the number of cubic centimetres of normal 



