10 BACTERIOLOGICAL EXAMINATION OF WATER. 



The gelatine plates should be made in the following manner : 

 Melt three gelatine tubes in a water-bath at a temperature 

 of about 30 C., sterilise the cotton- wool plugs in a Bunsen 

 flame and twist the plugs round so as to loosen them. Draw 

 up the required amount of water in a sterile pipette ; then 

 holding the pipette almost horizontally with the right hand, 

 take the tube containing the melted gelatine in the left 

 hand between the thumb and first finger and again sterilise 

 the plug. Now hold the tube as nearly horizontal as possible, 

 without permitting the gelatine to touch the wool ; remove 

 the plug with the third and fourth fingers of the right 

 hand, introduce the water, again sterilise the neck of the test- 

 tube, and replace the plug. Place the pipette on a sterile metal 

 stand and roll the test-tube, held vertically between the palms 

 of the hands, so as to thoroughly mix the water and gelatine. 

 The test-tube must not be shaken up and down or air-bubbles 

 will be introduced into the gelatine. Having thoroughly mixed 

 the water and gelatine, hold the test-tube in the right hand, 

 sterilise the wool and neck of the tube in the flame, and then 

 withdraw the plug with the inner margin of the left hand. Next 

 raise the lid of the Petri dish with the thumb, first and second 

 fingers of the left hand, introduce the neck of the test-tube into 

 the aperture, pour out the mixture of water and gelatine, replace 

 the lid and gently diffuse the contents into a thin layer over the 

 bottom of the dish. When making the plates it will be noticed 

 that it is almost impossible to pour out all the gelatine from the 

 test-tubes without opening the Petri dishes to a dangerous extent, 

 and as the portion of gelatine still remaining at the bottom of the 

 test-tube may contain a few organisms, it is best, after the plug 

 has been replaced, to distribute the gelatine over the walls of the 

 tube in as thin a layer as possible in fact, to convert the tube 

 into an Esmarch roll. The plates and test-tubes are usually 

 incubated at 20 to 22 C. and the colonies which develop are 

 counted from day to day. As a rule it will be found that there 

 is no very material increase in the number of colonies after the 

 fifth day, so if possible the plates should be kept for this 

 period. If many liquefying organisms are present the plates 

 will usually be destroyed by the third or fourth day. 



The simplest method of counting the number of colonies is to 



