The term 'density' refers to the proportion of an object's mass to it's volume (Grasse, 1996). The ratio between the density of a substance and the density of water is called specific gravity and is used in many diagnostic tests. ONe example is the specific gravity of urine which is normally determined as part of a laboratory evaluation of the health of an individual. The specific gravity of urine is normally 1.003 and 1.030. The density of urine is expected to be greater than the density of water because compounds such as salts and urea are dissolved in urine. If the specific gravity of a person's urine is too low or too high, a physician might suspect some kind of problem and other tests are usually conducted to pin down the real cause. Tyson, Grasse, "On Being Dense", Natural History, Jan 1996, v. 105, n 1, 66.