It can be shown, using a glass tube open at one end containing dry air trapped in the tube by a suitable liquid, that for a fixed mass of gas, the volume of the gas V is directly proportional to its temperature T provided its pressure is kept constant.V / T = constant If a graph of V is plotted against temperature in °C a straight line is obtained which does not go through the origin. However, the graph can be extrapolated to meet the temperature axis at -273.15°C or absolute zero (0K). The volume is directly proportional to the absolute temperature using the kelvin scale.