NUTRIENT MEDIA 59 



overheating, otherwise the potatoes will lose their natural 

 white colour, and become sodden in appearance. Potatoes 

 possess a slightly acid reaction, so the surface is rendered 

 faintly alkaline with sodium bicarbonate for the growth of 

 certain micro-organisms whose growth requires alkalinity. 



(15) Potato-Water Gelatine. — This medium is used for the 

 differential separation of the typhoid and coli bacilli. It is 

 prepared as follows : 500 grammes of potatoes are thoroughly 

 washed, peeled, finely grated, and squeezed through a linen 

 cloth by means of a small laboratory press. The opaque 

 juice is then heated (with or without treatment with a little 

 pure animal charcoal to decolorise) in the steam chamber 

 for one hour. It is then filtered, and 10 per cent, of ' gold- 

 leaf ' gelatine added to the clear fluid ; this is again heated 

 in the steam chamber, filtered if necessary, poured into 

 test-tubes, and sterilised on three successive days. 



(16) Potato-Water Gelatine (Eisner's modification). — This is 

 the same as above, with 1 per cent, of potassium iodide 

 added. The best way of preparation is to add a sterilised 

 solution of potassium iodide in requisite amount to the 

 potato-gelatine which has just been made ready for use. 



(17) Blood Serum. — Blood serum is an excellent medium 

 for all the pathogenic organisms, all of which grow with 

 greater rapidity on this medium than on any other. It 

 does not matter much what animal the blood is taken from, 

 but the blood of the horse yields the lightest-coloured 

 serum. Blood from horses, bullocks, pigs, and sheep is 

 generally to be obtained from slaughtermen. The blood 

 from the jugular vein or an incised wound is allowed to 

 run into a tall glass vessel. The vessel containing the 

 blood is at once placed in a cool place without the least 

 shaking, and allowed to stand overnight, when it will be 

 found that a firm clot has formed ; the clear serum is then 

 drawn off by means of a glass siphon or large pipette. The 



