206 BACTERIOLOGY. 



drawn out in a gas-flame. Both of these tubes must be 

 provided, before sterilization, with a plug of cotton ; 

 this is to prevent the access of foreign organisms to the 

 medium during manipulations. At the inner side of 

 the rubber stopper that is, the end which is to be 

 inserted into the test-tube the glass tubes are of 

 different lengths : one reaches to within 0.5 cm. 

 of the bottom of the test-tube, the other is cut off flush 

 with the under surface of the stopper. The outer end 

 of the longer glass tube is then connected with a 

 hydrogen generator and hydrogen is allowed to bubble 

 through the gelatin (Fig. 38, A) in the tube until all 

 contained air has been expelled and its place taken 

 by the hydrogen. 1 When the hydrogen has been 

 bubbling through the gelatin for about five minutes 

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



1 Before beginning the experiment it is always wise to test the hydro- 

 gen i. e., to see that it is free from oxygen and that 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 sulphuric acid of about 25 to 30 per cent, 

 strength. With the primary evolution 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, 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 rubber tubing. When the water has been replaced test the gas 

 by holding a flame near the open mouth of the test-tube. If no explo- 

 sion occurs, the hydrogen is safe to use. Should there be an explosion, 

 the generation of hydrogen must be continued in the apparatus until 

 it burns with a colorless flame when tested in a test-tube. 



