i6o PRACTICAL BACTERIOLOGY. 



The samples may be now sent by post, pp,cked up in 

 boxes. On no account should more than five test- 

 tubes be packed in the smaller tin. The first time 

 this operation is attempted it is probable that in one 

 respect or other some mistakes will be made^ but then 

 the student will learn to avoid them another time. 



Plate cultivations are now made from these speci- 

 mens, preferably in the double dishes described before. 

 These are sterilised in the hot-air steriliser, as well as 

 a sufficient number of pipettes, each of which has one 

 cubic centimetre graduated in tenths. Nutrient gela- 

 tine (10^ gelatine) is generally the most suitable nu- 

 trient medium, agar-agar is only to be recommended 

 when a high temperature is necessary. 



The fluid gelatine is poured into the double dishes, 

 which are immediately closed (the cover must not be 

 laid down meanwhile), and a small quantity of the 

 water sample is taken up with a sterilised pipette. If 

 there is no means of judging beforehand whether the 

 water contain many or few bacteria, it is best to pre- 

 pare three dishes. In the first of these 1 ccm. of 

 water is added to the gelatine, in the second \ ccm., 

 and in the third -5*5 ccm. The covers should always 

 be replaced as rapidly as possible, and then the gela- 

 tine and water are thoroughly mixed by moving the 

 dish, carefully from side to side. They are then laid 

 on as borizontal a surface as possible, until they become 

 cool, and the gelatine has solidified ; they may then 



