The gas is oxygen and can be proved in the following way. Close the test-tube with the thumb under water, and invert it over a dish containing a quantity of pyrogallate of potash (5 % pyrogallic acid to which an excess of caustic potash has been added). Then use a bent tube to introduce into the test-tube a quantity of this solution. The pyrogallate solution coming in contact with the gas will absorb it, and will, therefore, rise and completely fill up the test-tube. The pyrogallate solution absorbs oxygen. The gas in the tube is, therefore, oxygen.