198 BACTERIOLOGY. 



outer end of the longer glass tube is then connected 

 with a hydrogen generator and hydrogen is allowed 

 to bubble through the gelatin (Fig. 41, a) in the tube 

 until all contained air has been expelled and its place 

 taken by the hydrogen/ When the hydrogen has been 

 bubbling through the gelatin for about five minutes 

 (at least) one can be reasonably sure that all oxygen 

 has been expelled. The drawn-out portions of the 

 tubes can then be sealed in the gas-flame without fear 

 of an explosion. The protruding end of the rubber 

 stopper is then painted around with melted paraffin 

 and the tube rolled in the way given for ordinary 

 Esmarch tubes. A tube thus prepared and containing 

 growing colonies is shown in Fig. 41, B. 



The development that now occurs is in an atmos- 

 phere of hydrogen, all oxygen having been expelled. 

 During the operation the tube containing the liquefied 

 gelatin should be kept in a water-bath at a temperature 

 suflicieutly high to prevent its solidifying, and at the 



^ Before beginning the experiment it is always wise to test ttie liydrogen — 

 i. e., to see that it is free from oxygen and there is no danger of an explosion, 

 for unless this be done the entire apparatus may be blown to pieces and a 

 serious accident occur. The agents used should be pure zinc, and pure sul- 

 phuric acid of about 26 to 30 per cent, strength. With the primary evolu- 

 tion of the gas the outlet of the generator should be closed and kept closed 

 until the gas reservoir is quite filled with hydrogen. The outlet should then 

 be opened and the entire volume of gas allowed to escape, care being taken 

 that no flame is in the neighborhood. This should be repeated again, after 

 which a sample of the hydrogen generated should be collected in an inverted 

 test-tube in the ordinary way for collecting gases over water, viz., by filling a 

 test-tube with water, closing its mouth with the thumb, inverting it, and 

 placing its mouth under water, when, after removing the thumb, the water 

 will be kept in it by atmospheric pressure. The hydrogen which is flowing 

 from the open generator may be conducted to the test-tube by a bit of rubber 

 tubing. When the water has been replaced try the gas by holding a flame 

 near the open mouth of the test-tube. If no explosion occurs, the hydrogen 

 is safe to use. Should there be an explosion the generation of hydrogen must 

 be continued in the apparatus until it simply burns with a colorless flame 

 when tested in a test-tube. 



