These substances should be obtained from Kahlbaum (of Berlin); in the pure form, and when possible as large crystals, and the method of preparing a medium containing either of them may be exemplified by describing Dextrose Solution. ~Dextrose Solution.~-- 1. Weigh out Peptone 20 grammes Glucose 10 grammes and grind together in a mortar; then emulsify in 100 c.c. of distilled water heated to 60° C. 2. Place in a flask and add Distilled water 850 c.c. 3. Steam in the steamer at 100° C. for twenty minutes to dissolve the peptone and glucose. 4. Add Kubel-Tiemann litmus solution (Kahlbaum) 50 c.c. (The substances enumerated above react acid to phenolphthalein, but variously toward the neutral litmus solution. To such as react acid, add very cautiously n/1 sodium hydrate solution to the medium in bulk until the neutral tint has returned). 5. Fill into tubes in which have previously been placed the inverted Durham's gas tubes. 6. Sterilise in the steamer at 100° C. for _twenty minutes_ on each of three successive days. NOTE.--On no account should these media be sterilised in the autoclave, as temperatures above 100° C. themselves induce hydrolytic changes in the substances in question. It is equally important that the twenty minutes should not be exceeded in sterilisation, as neglect of this precaution may discolour the litmus or lead to the production of yellowish tints when the tubes are subsequently inoculated with acid-forming bacteria. ~Neutral Litmus Solution.~ The most satisfactory is the Kubel-Tiemann, prepared by Kahlbaum. It can however be made in the laboratory as follows: 1. Weigh out Commercial litmus 50 grammes, and place in a well stoppered 500 c.c. bottle; measure out and add 300 c.c. alcohol 95 per cent. 2. Shake well at least once a day for seven days--the alcohol acquires a green colour. 3. Decant off the green alcohol and fill a further 300 c.c. 95 per cent. alcohol into the bottle and repeat the shaking. 4. Repeat this process until on adding fresh alcohol the fluid only becomes tinged with violet. 5. Pour off the alcohol, leaving the litmus as dry as possible. Connect up the bottle to an air pump and evaporate off the last traces of alcohol. 6. Transfer the dry litmus to a litre flask, measure in 600 c.c. distilled water and allow to remain in contact 24 hours with frequent shakings. 7. Filter the solution into a clean flask and add one or two drops of pure concentrated sulphuric acid until the litmus solution is distinctly wine-red in colour. 8. Add excess of pure solid baryta and allow to stand until the reaction is again alkaline. 9. Filter. 10. Bubble CO_{2} through the solution until reaction is definitely acid. 11. Sterilise in the steamer at 100° C. for thirty minutes on each of three consecutive days. This sterilises the solution and also drives off the carbon dioxide, leaving the solution neutral.