From Big Medical Encyclopedia

DENSITY — the physical quantity determined for homogeneous bodies by the relation of body weight to unit of its volume. For the characteristic of heterogeneous bodies the concept «average density» — the relation of body weight to the volume occupied by him is entered. In a lab. to practice often use the concept «relative density» representing P.'s relation of two substances at certain reference conditions. For liquids and solid matters relative P. is usually expressed by P.'s relation of a body at t ° 20 ° to GT. a distilled water at t ° 4 °, to-ruyu it is possible to accept equal 1 g/cm 3 . For gases relative P. is defined in relation to P. of a dry air or hydrogen. P.'s unit in the SI system is the kg/m 3  ; often use P.'s unit in the SGS system (g/cm 3 ) and stand-alone units (g/ml, g/l etc.); at P.'s measurements with a margin error within 0,01% accept that 1 g/ml is approximately equal to 1 g/cm 3 .

Range of values P. of natural bodies and Wednesdays is extremely wide. So, P. of the interstellar environment does not exceed 10 - 21 kg/m 3 , and P. of neutron stars can reach 10, probably 20 kg/m 3 .

Average P. Zemli makes 5520 kg/m 3 , and the Sun — 1410 kg/m 3 .

For each chemically homogenous matter and for solutions at this temperature of P. is a constant. Therefore in many cases in size P. it is possible to judge concentration of substance in solution. During the studying of cellular plasma membranes, membranes of intracellular organellas, and also nucleinic to - t and other connections the methods of their allocation and division based on distinctions in their density are widely used (see. Centrifuging ).

The item of substance changes with change of temperature and pressure. Owing to a small compressibility coefficient liquid and solids (10 - 3 — 10 - 7 share of volume at change of pressure upon 1 atm) influence of pressure upon volume liquid and solids slightly, and to them is usually neglected. P.'s dependence on temperature is expressed stronger. Usually with fall of temperature of P. of substance increases, however in some cases in a certain interval of temperatures inverse relation is observed. In particular P. of water with fall of temperature to 4 ° increases, and then begins to fall. Therefore the ice which is formed at negative temperatures remains on a water surface, covering it as a fur coat and protects water organisms from overcooling. The possibility of existence of live organisms in severe climatic conditions of the Arctic and Antarctic is connected with this property of water. At phase transformations of substance P. changes jump. So, upon transition from liquid state to firm P. usually grows, but at water and cast iron it decreases.

By the item it is connected with other important physical parameters of substance — a pier. it is powerful (weight), a diffusion coefficient, viscosity, molecular refraction, polarizability, etc. therefore determination of these parameters can be made on the basis of definition of exact values P. of these or those substances.

In clinicodiagnostic and a dignity. - a gigabyte. laboratories the greatest distribution was gained by methods of measurement of P. with the help areometers (see) and densimeters — glass vessels with precisely measured volume. There are also other methods of definition of P. (see. Densimetriya ). P.'s measurement biol, liquids (urine, blood, cerebrospinal liquid) serves as diagnostic test at a number of diseases.

Bibliography: Biochemical research of membranes, under the editorship of E. Maddie, the lane with English, page 9, M., 1979; In about with to r e with e of N with to and y P. I. Tekhnika of laboratory works, M., 1973; And at z N of e r S. I. and d river. Measurement of weight, volume and density, M., 1972; The Reference book of the chemist, under the editorship of B. P. Nikolsky, etc., t. 1, L., 1971; F r and y f e of l - d e r. Physical biochemistry, the lane with English, page 332, M., 1980.

G.E. Fedorov.