I suspect that the gas was hydrogen.
Was Activity 1.7 the electrolysis of water?
Water decomposes into hydrogen and oxygen when you pass an electric current through it.
The chemical equation is
The hydrogen is given off at one electrode, and oxygen is given off at the other.
According to the equation, you get two molecule of hydrogen for every one molecule of oxygen.
So you should get twice as much gas in one tube as in the other, and the gas with the greater volume will be hydrogen.