The weight-density of most substances on Earth is a simple ratio of weight to volume: The weight-density of water for example is 62.42 lbf per cubic foot, and is quite constant. Other substances are generally more, or less dense than water, and that's one way we have of identifying them: By comparing them to the standard density of water. Substances more dense than water will sink in it, and substances less dense than water will float on it: For fluids the easiest way to check their density is to use a hydrometer; thats how the charge of a battery is checked. When it's charged the electrolyte contains more acid; that increases its density and the hydrometer's bulb floats higher; giving a measure of the charge. In theory at least we can neglect air's [slight] bouyancy effect, and weigh a certain volume of a particular substance on a weight-scale; then dividing its weight by the [standard] weight of the same volume of water; the quotient is the specific gravity - or density _Relative_ to water - of that particular substance. In practice it's often not very easy to accurately determine the volume of an _object_; body or mass of a material substance. That's why _Tables_ of weight-density and specific gravity were painstakingly worked out for most substances. In using materials much heavier and more dense than water for artifacts to serve as standard counter-weights; we start running into the problem of loosing accuracy due to neglecting air bouyancy: The weight-density of platinum is more than twenty times that of water! If the metric system is to be really accurate, they'll have to address this problem with bouyancy; unless they want to do all weighing in a vacuum.