ient bouillon.

~Potato Gelatine (Elsner).~--

1. Take 1 kilo of potatoes, wash thoroughly in water, peel, and finally
grate finely on a bread-grater.

2. Weigh the potato gratings, place them in a 2-litre flask, and add
distilled water in the proportion of 1 c.c. for every gramme weight of
potato. Allow the flask to stand in the ice-chest for twelve hours.

3. Strain the mixture through butter muslin, and filter through Swedish
filter paper into a graduated cylinder.

4. Add 15 per cent. gelatine to the potato decoction and bubble live
steam through the mixture for ten minutes.

5. Estimate the reaction; adjust the reaction of the medium mass to +25.

6. Cool the medium to below 60 deg. C.; clarify with egg as for nutrient
gelatine (_vide_ page 166).

7. Add 1 per cent. potassium iodide (powdered) to the medium.

8. Filter through papier Chardin.

9. Tube and sterilise as for nutrient gelatine.

~Aesculin Agar.~--(B. coli and allied organisms give black colonies
surrounded by black halo.)

1. Measure out 400 c.c. distilled water into a tared 2-litre flask.

2. Weigh out

Agar 15 grammes
Peptone 10 grammes
Sodium taurocholate 5 grammes

and make into a thick paste with 150 c.c. distilled water.

3. Add this paste to the distilled water in the flask.

4. Dissolve the ingredients by bubbling live steam through the mixture.

5. Weigh out

Aesculin 1.0 gramme
Ferric citrate 0.5 gramme

and dissolve in a second flask containing 100 c.c. distilled water.

6. Mix the contents of the two flasks--adjust the weight to the
calculated medium figure (in this case 1031.5 grammes) by the addition
of distilled water at 100 deg. C.

7. Clarify with egg and filter.

8. Tube and sterilise as for nutrient agar.

~Bile Salt Agar (MacConkey).~--

1. Weigh out powdered agar, 15 grammes (= 1.5. per cent.), and emulsify
with 200 c.c. _cold tap_ water.

2. Weigh out peptone, 20 grammes (= 2 per cent.), and emulsify with 200
c.c. _tap_ water previously warmed to 60 deg. C.

3. Mix the peptone and agar emulsions thoroughly.

4. Weigh out sodium taurocholate, 5 grammes (= 0.5 per cent.), dissolve
it in 300 c.c. _tap_ water, and use the solution to wash the
agar-peptone emulsion into a tared 2-litre flask.

5. Bubble live steam through the mixture for twenty minutes.

6. Adjust the weight of the medium mass to the calcula