CULTIVATION WITHOUT OXYGEN. 189 



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. 36, A) in the tube until all contained 

 air has been expelled and its place taken by the hydro- 

 gen. 1 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. 36, B. 



The development that now occurs is in an atmosphere 

 of hydrogen, all oxygen having been expelled. During 



1 Before beginning the experiment it is always wise to test the hydrogen, 

 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 seri- 

 ous accident occur. The agents used should be pure zinc, and pure sulphuric 

 acid of about 25 to 30 per cent, strength. When the gas is beginning to be 

 given off, 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 conveyed 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 con- 

 tinued in the apparatus until it simply burns with a colorless flame when 

 tested in a test-tube. 



