808 METHODS OF INVESTIGATING BACTERIA. 



of a spirit lamp. When these bulbs are brought back to 

 the laboratory they are again disinfected and washed with 

 sterilised water. A mark is made with a file close to the 

 junction of the tube with the bulb (at c), and the tube 

 broken off at this point. The resulting opening is suffi- 

 cient to enable one to remove the desired amount of 

 water a drop to a cubic centimetre or more by means 

 of a sterilised pipette. 



Investigation. The best method of ascertaining the number and 

 species of the germs present is by means of gelatine 

 plates, or by Esmarch's method. In order to obtain 

 useful plates (containing 10 to 5,000 colonies), it is well 

 to make the first experiment with from 1 to 10 drops of 

 water ; according to the result of this experiment we can 

 employ in the subsequent attempts either larger quantities, 

 or we can dilute the water with sterilised distilled water. 

 If, however, we cannot again obtain a fresh specimen of the 

 water to be tested we mast at first make a large number 

 of plates containing varying amounts of the water. 



Error as the It is of importance to examine the water as soon 



result of keep- . . 



ing a specimen as possible, at any rate within one to three hours after it 

 has been obtained. This precaution is essential, in view 

 cf the fact that has now been confirmed by many in- 

 vestigators that the bacteria in the water multiply rapidly 

 if it is kept in a warm place (see p. 711). In trans- 

 porting the water it should be packed in ice; Wolffhiigel 

 has indeed ascertained that when water is preserved in 

 ice the number of living germs present gradually di- 

 minishes ; but the diminution is not so marked during 

 the first twenty-four or forty-eight hours as to make any 

 essential difference in the result. The great point is to 

 avoid as far as possible any delay in the investigation. 



Fol and Dunant have described a method of analysing 

 water which is closely similar to Miguel's method of 

 examining air, but it also possesses all the errors of this 

 method, and is not of any practical use.* 



In judging the results of the investigation we have to 

 bear in mind the points which have been referred to on 

 p. 714. 



* See Boltoi, Ztitschr.f. Hygiene, vol. 1, Part 1. 



