6. Filter through papier Chardin, using the hot-water funnel.

7. Add sterile litmus solution, sufficient to colour the medium a pale lavender.

8. Tube, and sterilise as for nutrient agar.

~Glycerine Potato Bouillon.~--

1. Take 1 kilo of potatoes, wash thoroughly in water, peel, and 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 b.u.t.ter muslin and filter through Swedish filter paper into a graduated cylinder. Note the amount of the filtrate.

4. Place the filtrate in a flask, add an equal quant.i.ty of distilled water, and heat in the steam steriliser for sixty minutes.

5. Add glycerine, 4 per cent., mix thoroughly, and again filter.

6. Tube and sterilise as for nutrient 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 b.u.t.ter 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 ma.s.s to +25.

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

7. Add 1 per cent. pota.s.sium 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 100C.

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 60C.

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 ma.s.s to the calculated figure for one litre (1040 grammes).

7. Cool to 60 C. and clarify with egg as for nutrient agar (_vide_ page 168).

8. Filter through papier Chardin, using the hot-water funnel.

9. Weigh out lactose, 10 grammes (= 1 per cent.), and dissolve it in the agar.

If desired, add 5 c.c. of a 1 per cent. (= 0.5 per cent.) aqueous solution of neutral red.

10. Tube, and sterilise as for nutrient agar.

~Litmus Nutrose Agar (Drigalski-Conradi).~--

This medium should be prepared in precisely the same manner as the Nutrose agar described on page 172 subst.i.tuting meat extract for serum water, and increasing the percentage of agar added per litre to 3 per cent.

~Fuchsin Agar (Braun).~--

1. Liquefy and measure out into a sterile flask: