CHANGES IN THE REACTION OF MEDIA 211 



pass two glass tubes. These must all have been sterilized 

 in the steam sterilizer before using. On the outer side of 

 the stopper these two tubes are bent at right angles to the 

 long axis of the test-tube into which they are to be placed, 

 and both are slightly 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.^ "When the hydrogen has 

 been bubbling through the gelatin for about five minutes 

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



^ 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. 



