178 BACTERIOLOGV. 



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. 35, b. 



The development that now occurs is in an atmosphere 

 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 suffi- 

 ciently high to prevent its solidifying and at the same 

 time not high enough to kill the organisms with which 

 it has been inoculated. 



One of the obstacles to the successful performance of 

 this method is the bubbling of the gelatin, the foam 

 from which will often fill the exit tube and sometimes be 



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

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

 unless this is 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. 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 process must be repeated until the hydrogen simply 

 burns with a colorless flame. 



