And these guys will evaporate and will create a certain vapor pressure. The vapor pressure is this the after picture? After we add the volatile compound, we're going to have a mixture of the new compound and the old compound found on the surface area. But now the red guys, the new compound, are allowed to evaporate. So now the vapor pressure will be due to these molecules and these molecules. So Roth's law will tell us that the final vapor pressure of our system will be the vapor pressure due to the new guys, plus the vapor pressure due to the old guys.