According to the law of Avogadro, equal volumes of different gases under the same conditions of temperature and pressure contain equal numbers of molecules; therefore, since the density depends upon the number of molecules present in unit volume, it follows that for a comparison of the densities of gases, the determinations must be made under coincident conditions, or the observations reduced or re-computed for coincident conditions.
Biltz (Ber., 1888, 21, p. 2013; 1901, 34, p. 2490) showed that the vapour density decreased with the temperature, and also depended on the pressure.
There is, however, a tendency for people to remain rooted to the 2 See maps of density of population in Bartholomew's great largescale atlases, Atlas of Scotland and Atlas of England.
One of the most remarkable of Airy's researches was his determination of the mean density of the earth.
Density of population is measured by the average number of people residing on a unit of area; but in order to compare one part of the world with another the average should, strictly speaking, be taken for regions of equal size or of equal population; and the portions of the country which are permanently uninhabitable ought to be excluded from the calculation.'
The density of solid sulphur is 2 062 to 2'070, and the specific heat 0.1712; it is a bad conductor of electricity and becomes negatively electrified on friction.
As Lord Kelvin has shown (Baltimore Lectures, p. 304, 1904) (16) may also be obtained by the consideration of the mean density of the altered medium.
From this he was led to the final value of 6.566 for the mean density of the earth as compared with that of water (Phil.
A solution of the free acid may be obtained by decomposing the barium salt with dilute sulphuric acid and concentrating the solution in vacuo until it attains a density of about 1.35 (approximately), further concentration leading to its decomposition into sulphur dioxide and sulphuric acid.
Of trespassers the number killed per mile of line is about as large in England as in America, the density of population and of traffic in Great Britain apparently counterbalancing the laxity of the laws against trespassing in America.
Henry Cavendish, from which it appeared that Cavendish, already famous by many other researches (such as the mean density of the earth, the composition of water, &c.), must be looked on as, in his day, a man of Maxwell's own stamp as a theorist and an experimenter of the very first rank.
His terminology was vague and provoked caustic criticism from Berzelius; he assumed that all molecules contained two atoms, and consequently the atomic weights deduced from vapour density determinations of sulphur, mercury, arsenic, and phosphorus were quite different from those established by gravimetric and other methods.
The oxychloride, bromides, and other compounds were subsequently discovered; here we need only notice Moissan's preparation of the trifluoride and Thorpe's discovery of the pentafluoride, a compound of especial note, for it volatilizes unchanged, giving a vapour of normal density and so demonstrating the stability of a pentavalent phosphorus compound (the pentachloride and pentabromide dissociate into a molecule of the halogen element and phosphorus trichoride).