In the first part of this procedure, you will be setting up the gas collection apparatus. In the process of growing, yeast produce carbon dioxide (CO2), and if the number of living yeast decreases, the amount of CO2 produced will also decrease. Consequently, measuring CO2 production is a good way to determine how much yeast is alive relative to the other conditions you test. The gas collection apparatus that you set up here will show you how much CO2 is being produced by the yeast.