74 PRACTICAL BACTERIOLOGY. 



from eacaping and becoming replaced by air, which 

 would render the experiment useless. 



The test-tubes containing gelatine are now placed 

 in a vessel containing water at 37°-40° C, in order to 

 prevent the solidification of the gelatine during the 

 introduction of the hydrogen ; should this however 

 occur accidentally before they are rolled, the gelatine 

 can be reliquefied by placing the tubes in warm water 

 at about 40° C. The stopcock of the generating appa- 

 ratus is then opened, and the hydrogen is allowed to 

 pass through the fluid gelatine until all the air is forced 

 out of the test-tube ; this will take place in about five 

 minutes. It is well to allow rather a longer time in the 

 case of the first test-tube, on account of the air which 

 is present in the tubes, etc., and which is carried along 

 by the hydrogen. When all the air has been expelled 

 from the gelatine and test-tube, the tubes are quickly 

 melted off, first the one open to the air, and then the 

 one connecting the test-tube with the generating 

 apparatus, so that the test-tube is hermetically sealed 

 up from the air. 



The gelatine is now rolled out, either in the manner 

 described first, i.e., the mouth of the test-tube being 

 placed over the edge of the vessel, in which case the 

 latter must be high enough to prevent the melted off 

 ends of the glass tubes from touching the table, or by 

 rolling the test-tube freely in the hands under a stream 

 of water from a tap. 



