The ratio of the mass of any substance to the volume that it occupies. Density is expressed in kg m3. The density of the matter constituting the glacier can range from as low as 10 kg m3, at the surface in unusual weather, to the density of pure ice at depths at which all air has been squeezed out of bubbles. It is very common to assume that the bulk density of the glacier is 900 kg m3. This reduced density is a rough-and-ready allowance for the presence of snow and firn, large voids (crevasses, moulins and subglacial cavities) and sediment. Where a large proportion of the glacier thickness consists of snow and firn, a bulk density even lower than 900 kg m3 is appropriate. Where there is relatively little snow or firn, and the temperature is very low, a higher density, approaching or even exceeding the conventional 917 kg m3, may be appropriate. In studies of mass balance, however, densities are never known with the accuracy of laboratory measurements of pure ice, which are made by measuring the lattice parameters of single crystals. Typical field instruments are hand-held corers and spring balances, and inaccuracies of the order of 48% are usual. Better accuracy is possible in principle with advanced devices such as neutron-scattering probes, but these are not in routine use. In some circumstances, such as when a load of low-density snow produces compensating densification at depth, the density of the mass gained or lost by the glacier may be assumed equal to the bulk density. See Sorge's law.