_The Preparation of Nutrient Gelatine and Agar-agar._--Take half a kilogramme (one pound) of beef as free as possible from fat, chop finely, transfer to a flask or cylindrical vessel, and shake up well with a litre of distilled water. Place the vessel in an ice-pail, or ice-cupboard, or in winter in a cold cellar, and leave for the night. Next morning commence with the preparation of all requisite apparatus. Thoroughly wash and rinse with alcohol about 100 test-tubes, and allow them to dry. Plug the mouths of the test-tubes with cotton-wool, place them in their wire cages in the hot-air steriliser, to be heated for an hour at a temperature of 150°C. In the same manner cleanse and sterilise several flasks, and a small glass funnel. In the meantime, the meat infusion must be well shaken, and the liquid portion separated by filtering and squeezing through a linen cloth or a meat press. The red juice thus obtained must be brought up to a litre by transferring it to a large measuring glass and adding distilled water. It is then poured into a sufficiently large and strong beaker, and set aside after the addition of ten grammes of peptone, five grammes of common salt, and 100 grammes of best gelatine. In about half an hour the gelatine is sufficiently softened, and subsequent heating in a water bath causes it to be completely dissolved. The next process requires the greatest care and attention. Some micro-organisms grow best in a slightly acid, others in a slightly alkaline, medium. For example, for the growth and characteristic appearances of the _comma bacillus_ of Asiatic cholera a faintly alkaline soil is absolutely essential. This slightly alkaline medium will be found to answer best for most micro-organisms, and may be obtained as follows:--With a clean glass rod dipped in the mixture, the reaction upon litmus-paper may be obtained, and a concentrated solution of carbonate of soda must be added drop by drop until red litmus-paper becomes faintly blue. If it is too alkaline, it can be neutralised by the addition of lactic acid. Finally, the mixture is heated for an hour in the water-bath. Ten minutes before the boiling is completed the white of an egg beaten up with the shell is added, and the liquid is then filtered while hot.