The density, of a substance is its mass per unit volume. The symbol most used for density is ρ, although the Latin letter D can be used. Mathematically, density is defined as mass divided by volume: ρ = m V where ρ is the density, m is the mass, V is the volume. In some cases, density is loosely defined as its weight per unit volume, although this is scientifically inaccurate – this quantity is more called specific weight. For a pure substance the density has the same numerical value as its mass concentration. Different materials have different densities, density may be relevant to buoyancy and packaging. Osmium and iridium are the densest known elements at standard conditions for temperature and pressure but certain chemical compounds may be denser. To simplify comparisons of density across different systems of units, it is sometimes replaced by the dimensionless quantity "relative density" or "specific gravity", i.e. the ratio of the density of the material to that of a standard material water.

Thus a relative density less than one means. The density of a material varies with pressure; this variation is small for solids and liquids but much greater for gases. Increasing the pressure on an object decreases the volume of the object and thus increases its density. Increasing the temperature of a substance decreases its density by increasing its volume. In most materials, heating the bottom of a fluid results in convection of the heat from the bottom to the top, due to the decrease in the density of the heated fluid; this causes it to rise relative to more dense unheated material. The reciprocal of the density of a substance is called its specific volume, a term sometimes used in thermodynamics. Density is an intensive property in that increasing the amount of a substance does not increase its density. In a well-known but apocryphal tale, Archimedes was given the task of determining whether King Hiero's goldsmith was embezzling gold during the manufacture of a golden wreath dedicated to the gods and replacing it with another, cheaper alloy.

Archimedes knew that the irregularly shaped wreath could be crushed into a cube whose volume could be calculated and compared with the mass. Baffled, Archimedes is said to have taken an immersion bath and observed from the rise of the water upon entering that he could calculate the volume of the gold wreath through the displacement of the water. Upon this discovery, he leapt from his bath and ran naked through the streets shouting, "Eureka! Eureka!". As a result, the term "eureka" entered common parlance and is used today to indicate a moment of enlightenment; the story first appeared in written form in Vitruvius' books of architecture, two centuries after it took place. Some scholars have doubted the accuracy of this tale, saying among other things that the method would have required precise measurements that would have been difficult to make at the time. From the equation for density, mass density has units of mass divided by volume; as there are many units of mass and volume covering many different magnitudes there are a large number of units for mass density in use.

The SI unit of kilogram per cubic metre and the cgs unit of gram per cubic centimetre are the most used units for density. One g/cm3 is equal to one thousand kg/m3. One cubic centimetre is equal to one millilitre. In industry, other larger or smaller units of mass and or volume are more practical and US customary units may be used. See below for a list of some of the most common units of density. A number of techniques as well as standards exist for the measurement of density of materials; such techniques include the use of a hydrometer, Hydrostatic balance, immersed body method, air comparison pycnometer, oscillating densitometer, as well as pour and tap. However, each individual method or technique measures different types of density, therefore it is necessary to have an understanding of the type of density being measured as well as the type of material in question; the density at all points of a homogeneous object equals its total mass divided by its total volume. The mass is measured with a scale or balance.

To determine the density of a liquid or a gas, a hydrometer, a dasymeter or a Coriolis flow meter may be used, respectively. Hydrostatic weighing uses the displacement of water due to a submerged object to determine the density of the object. If the body is not homogeneous its density varies between different regions of the object. In that case the density around any given location is determined by calculating the density of a small volume around that location. In the limit of an infinitesimal volume the density of an inhomogeneous object at a point becomes: ρ = d m / d V, where d V is an elementary volume at position r

Rudnik, Cieszyn County

Rudnik is a village in Gmina Hażlach, Cieszyn County, Silesian Voivodeship, southern Poland. It has a population of 479; the name of the village is derived from a name of a local stream Rudnik, mentioned as early as 1442, a transformation of a word rudy, meaning rdzawy. It lies in the historical region of Cieszyn Silesia. Up to it was stated that the village was first mentioned in 1566 as Rudnik, however another documents exists issued by Wenceslaus III Adam, Duke of Cieszyn on 5 November 1608 which retrospectively affirms another document from 1523 that mentioned the village among others obliged to consume ale produced only in Cieszyn. Politically the village belonged to the Duchy of Teschen, a fee of the Kingdom of Bohemia, which after 1526 became part of the Habsburg Monarchy. After World War I, the fall of Austria-Hungary, the Polish–Czechoslovak War and the division of Cieszyn Silesia in 1920, it became a part of Poland, it was annexed by Nazi Germany at the beginning of World War II. After the war it was restored to Poland.

Rudnik lies in the southern part of Poland, 11 km north-east of the county seat, Cieszyn, 25 km west of Bielsko-Biała, 55 km south-west of the regional capital Katowice, 8 km east of the border with the Czech Republic. The village is situated on the geographical border between Ostrava Basin in the east and Oświęcim Basin in the west, between 260–280 m above sea level, 16 km north-west of the Silesian Beskids, it is drained by several streams, left tributaries of the Knajka, in the watershed of Vistula

Sergiyevsky District

Sergiyevsky District is an administrative and municipal district, one of the twenty-seven in Samara Oblast, Russia. It is located in the north of the oblast; the area of the district is 2,720 square kilometers. Its administrative center is the rural locality of Sergiyevsk. Population: 47,548; the population of Sergiyevsk accounts for 18.2% of the district's total population. Самарская Губернская Дума. №179-ГД 18 декабря 2006 г. «Устав Самарской области», в ред. Закона №6-ГД от 11 января 2016 г. «О внесении изменений в Устав Самарской области». Вступил в силу 1 января 2007 г. Опубликован: "Волжская коммуна", №237, 20 декабря 2006 г.. Самарская Губернская Дума. Закон №189-ГД от 28 декабря 2004 г. «О наделении статусом городского округа и муниципального района муниципальных образований в Самарской области», в ред. Закона №23-ГД от 30 марта 2015 г. «Об осуществлении местного самоуправления на территории городского округа Самара Самарской области». Вступил в силу по истечении десяти дней со дня официального опубликования.

Опубликован: "Волжская коммуна", №247, 31 декабря 2004 г.. Самарская Губернская Дума. Закон №45-ГД от 25 февраля 2005 г. «Об образовании городского и сельских поселений в пределах муниципального района Сергиевский Самарской области, наделении их соответствующим статусом и установлении их границ», в ред. Закона №106-ГД от 11 октября 2010 г. «О внесении изменений в законодательные акты Самарской области, устанавливающие границы муниципальных образований на территории Самарской области». Вступил в силу по истечении десяти дней со дня официального опубликования. Опубликован: "Волжская коммуна", №36, 28 февраля 2005 г