 So, moving on from the equations I talk about, the bulk density and the unit weight, what we can define now is something called the specific gravity. Now, the specific gravity, sometimes referred to as the particle density in some standards, we can give it the symbol G subscript S. The specific gravity is essentially visualized as if I had a soil and I plucked out a grain of that soil, so the grain looked like that, and from that grain I managed to core out a small cylinder of material, so out of that I took a small cylinder of material of non-volume and I put that on a balance and I measured its mass and I divided it by its volume. The third we measuring is essentially the mass of the solid and I'll be dividing it by the volume that that little mass took up, so the mass of the solid or the volume of the solid, this little bit of solid by the way, not the whole solid, and what I'd get out is the density of the solid, so that's, so the soil is made up of a whole range of different minerals and organic matter that all have a range of densities, so some minerals are denser than others and organic matter is less dense than those usually. So this is really just the average solid density that we have across, the average across all of these minerals in organic matter, but if I take that solid density and I divide it by the density of water, what I'd be defining is the specific gravity, it's really saying how many times more dense than water your solid is in your soil, so from this table you can see that different materials have different specific gravities, things like your metals iron and steel and gold have quite high specific gravities and rocks and minerals have slightly lower specific gravities than that and organic matter has lower even still, so you can see that soils are made up of a range of different materials and they'll express a specific gravity that's an average of those of its constituent parts, so soils tend to have specific gravity somewhere around two and a half to three. So a common misconception with specific gravity is that it's confused with the dry density, so we've already spoken about what the dry density is, what I'd like you to do is go back to the previous video and go through this video again and make sure that you're clear in your mind that what the difference between specific gravity and dry density is.