BACTERIA IN AIR, EARTH, AND WATER. 389 



lowered, then, when it has reached the required depth, the stopper is 

 removed, the bottle is allowed to fill, after which it is then drawn to the 

 surface, the water so obtained being at once transferred to a number of 

 smaller bottles prepared as above. 



Quite recently an exceedingly convenient apparatus for the collection of 

 water has been described by Dr. Pfuhl. It consists of a tall glass vessel with 

 a flat bottom 2. 5 centimetres in diameter and 10 centimetres long, with a 

 glass tube 6 or 8 centimetres long, which can be easily closed, leading from 

 this vessel. To prepare this it is only necessary to sterilize by heating in 

 a flame, and while the air is rarefied to seal up the point of the tube by 

 heat. When this has to be filled it is plunged under water or in the running 

 stream from a tap or pump, the tip of the tube is broken off with a pair of 

 sterilized forceps, water rushes in and about half fills the vessel. The outer 

 surface of the tube is then carefully dried with blotting-paper, gently 

 warmed to drive away the moisture from the glass near the opening, and 

 finally sealed as before in a spirit lamp flame. After it is thoroughly cooled . 

 the flask is well shaken, and should there be any leak this is made good. For 

 transport these flasks are packed in zinc cases with cotton wadding and ice. 

 To remove the water the tube is nicked with a file, broken off, and a steri- 

 lized pipette is introduced. In all cases, however, there is the difficulty of 

 transport, and it is a great matter if plate cultivations can be made at the 

 time that the water is drawn. Petri gets over this difficulty by making 

 his cultivations in double glass dishes, which are kept in position by india- 

 rubber bands ; and various other pieces of apparatus have been devised, 

 perhaps the best of these being Petruschky's flask, which can be used for the 

 cultivation of either serobic or anserobic organisms. This consists of a thin 

 flask flattened on two sides with an indentation at the neck to prevent the 

 flowing out of the softened gelatine. It may be used simply with a plug of 

 cotton wool. The mixture of gelatine and water is poured in and then allowed 

 to settle on one of the flat sides. For anserobic cultures, however, a couple 

 of glass tubes similar to those used in a wash bottle are introduced through 

 openings in an indiarubber cork. Hydrogen is driven through the bottle to 

 displace the air, and the ends of these tubes are carefully sealed, the flask 

 is laid on its side, and the gelatine is allowed to cool. These flasks are so 

 constructed that a microscopical examination may be made through the thin 

 glass walls, especially if care be taken to obtain a layer of gelatine or agar 

 sufficiently thin and of equal thickness throughout. Measure the size of 

 the drop delivered by a pipette in the following manner. Weigh a filter 

 paper on a fine balance, then put a gramme weight into the opposite scale, 

 and drop by drop deliver exactly one gramme of water on to the filter paper, 

 counting the drops as this is done ; then mark the pipette with the number 

 of drops that it delivers per gramme, after which it may be used for measur- 

 ing the water in making gelatine plate cultivations. It is now sterilized in 

 the hot air chamber at 150° C, or by being thoroughly washed out with bi- 

 chloride of mercury, then with distilled water that has been boiled and 

 allowed to cool, and then with absolute alcohol, the last traces of alcohol 

 being driven out by the heat of a spirit lamp. If a very large number of 

 organisms are present asingle small drop of the water, corresponding to about 

 the fiftieth part of a gramme or cc. will be sufficient, whilst a larger drop, 

 the twentieth part of a gramme, or even six or eight of these drops, may 

 have to be used in order to obtain a sufficient number of organisms in a 

 plate cultivation. Then prepare a number of glass plates in the following 



