ACID FUCHSINE. 



273 



After many trials, says Dr. Penny, it was found that the most 

 sensitive of the pink media was one that was exactly neutral and 

 contained acid fuchsine in the proportion of i to 25,000 or i to 

 33,000, those decolourised having an alkalinity equal to o'oo6 

 of caustic potash in every hundred cubic centimetres. 



Neutral peptone bouillon is prepared in the usual way, and 

 titrated for sodium chloride, so that it contains 0-5 centigramme 

 to the litre. This is important, as an increase of the salt propor- 

 tionately diminishes the growth of the bacteria, and hence interferes 

 with the reaction. After the bouillon is prepared the acid fuchsine 

 in aqueous solution is added, so that the medium contains the 

 fuchsine in the proportion of i to 25,000 for the pink or neutral 

 bouillon. The decolourised or alkaHne bouillon is prepared by 

 adding to every hundred cubic centimetres of the neutral medium 

 o-ooo6 of a milligramme of caustic potash and acid fuchsine in 

 the proportion of i to 33,000. 



After the addition of the fuchsine the bouillon is boiled for 

 about half-an-hour, and then filtered, put into tubes, each contain- 

 ing ten cubic centimetres, and then sterilised in the usual way. 

 It is observed that on heating the media the colour deepens, but 

 on cooling the original colour returns ; but it is sometimes paler. 



Dunham's solution of peptone, with six-per-cent. glycerin 

 containing acid fuchsine in the proportion of t to 33,000, has 

 been found to be the best adapted for distinguishing the intestinal 

 organisms. The decolourised solution is made in the same 

 manner as for the decolourised bouillon ; the advantage of this 

 medium over others is that, on account of its absence of colour, 

 it indicates the slightest trace of acid. 



Instead of Dunham's peptone solution, somatose has also been 

 employed in the same manner, adding to its solutions the same 

 amount of salt and glycerin ; it is not so satisfactory as Dunham's 

 solution, owing to the deep orange tint of the solution, and before 

 the proper colour was obtained the amount of acid fuchsine had 

 to be increased, so that it contained it in the proportion of i to 

 2,500. Decolourised solutions are prepared in the same manner 

 as others. 



Solid media of agar-agar or gelatin are prepared from Dunham's 

 or somatose solutions ; the glycerin, however, should not be 



