62 SEWAGE AND ITS PURIFICATION 



0*01 c.c. of the original) is transferred to No. 2 flask, and so on. 

 In this manner minute fractions of a c.c. of the original can be 

 taken with great accuracy, provided that each dilution is well 

 shaken so as to evenly distribute the bacteria. 



Nutrient Media. — Cultivations are made with various media, 

 such as nutrient gelatine, agar-agar, meat broth, milk, blood- 

 serum, potatoes, albumin, etc. The most important of these 

 is the nutrient gelatine, which consists of meat broth mixed 

 with 10 to 15 per cent, of gelatine, i per cent, of peptone, and 

 0*5 per cent, of common salt ; it is rendered neutral or very 

 faintly alkaline, and clarified with egg-albumin. While hot, 

 quantities of about 10 c.c. are run into test-tubes fitted with 

 cotton-wool plugs, the cotton-wool and tubes having been 

 previously sterilized by heat. These tubes are then fractionally 

 sterilized' by steaming for half an hour on three successive 

 days. When properly prepared the jelly is quite bright, should 

 not melt at 22° C, and should undergo no alteration on keeping, 

 as the cotton-wool plugs, while admitting air, exclude the 

 micro-organisms floating in it. Agar-agar is prepared in a 

 similar manner, ij to 2 per cent, of agar being substituted for 

 the gelatine ; this remains solid at blood heat, and is therefore 

 used for cultures at the higher temperatures. Tubes of gelatine 

 and agar are always stocked, and are employed for the following 

 cultivations : 



I. Plate cultures, originally devised by Koch, are almost 

 invariably resorted to for the isolation of bacteria. A gelatine 

 tube is melted at a temperature of about 30° C, i c.c. of 

 the sufficiently diluted sewage is added with a pipette, the tube 

 is gently shaken, and the contents poured into a shallow glass 

 dish with a close-fitting lid, this " Petri dish " and the pipette 

 having been previously sterilized in a hot-air sterilizer. The 

 gelatine is now allowed to set, the Petri dish being placed on a 

 levelled plate, incubated at 20° to 22° C, and examined from 

 day to day. If the sewage has been properly diluted, after 

 about forty-eight hours, according to the temperature, a number 

 of centres of growth become visible in the gelatine. These 

 ** colonies," which are due to proliferation of single scattered 

 organisms, will usually consist of pure cultures of the original 

 germ, and soon exhibit characteristic differences. Some form 

 cup-shaped depressions of liquid, others refuse to liquefy the 

 gelatine. The colonies may be either raised above the surface 

 or penetrate deeply into the gelatine ; their outline may be 



