 Okay, in the next set of videos we're going to be talking about the classification of soils and building up from first principles some of the parameters that we use to describe soils. So we've known pretty much since antiquity that soils have a range of different characteristics but it's only relatively recently that we've formalized that description. So what we need to do really to be able to describe soils in a meaningful way in a way that can be modeled is to take the huge range of complexity that exists and bring that complexity down into a simple model and the first step is this three phase model. So we take that complexity that exists in soils and we just describe soils in three phases. So we describe soils as having a solid phase which is all of the mineral matter, all of the organic matter that exists in soils. So that's our solid phase. Soils also have a liquid phase in this three phase model and from all soils that's just water. And soils also have a gas phase. So for more soils that's just air. So what do we know about these three phases? Well, we know that they all have a mass and they all have a volume. So we can describe these three phases in terms of their mass and volume. So let's get rid of this and let's talk about their mass. So the solid phase has a mass and we can describe that in terms of a parameter. So instead of writing mass of solid we can just write M for mass and subscript S to donate the solid phase. The liquid phase has a mass, capital M and we give it subscript W, mass of water. And the gas phase has a mass and the mass of air. Now the assumption that we make in soil mechanics is that mass of air is equal to zero. Obviously it has mass but relative to the other two phases that's pretty much true. So we assume that the mass of the air is zero. Okay, so they have a mass. We can also describe their volume. So the solid has a volume, volume S, or the volume of the solid. The liquid has a volume, VW, the volume of water and the air has a volume, the volume of air. The volume of the air and the volume of the water make up together the volume of the voids. So the void space within soils. So this together is the volume of the voids. So we've now got a three phase model of soils where we can describe the mass and the volume. So what we can do next is start taking those basic properties of soil and start building up some more complex parameters or some more useful parameters.