Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/481

Rh ATOMIC WEIGHTS.] CHEMISTRY ture and pressure, the temperature and pressure chosen being that at which the gases moat closely approximate to the requirements of the laws abovs stated, an estimate of the relative weights of their molecules is obtained. For example, the density of nitrogen referred to hydrogen is 14, since a given volume of nitrogen is found to weigh 14 times as much as an equal volume of hydrogen at the same temperature and under the same pressure; hence, according to Avogadro s hypothesis, the molecules of nitrogen are fourteen times as heavy as the hydrogen molecules. In the formation of hydrochloric acid gas equal volumes of chlorine and hydrogen unite without condensation. The density of chlorine gas referred to hydrogen is 35 - 36, and the simplest possible hypothesis of the composition of hydrochloric acid is that it consists of an atom of hydrogen weighing 1, and an atom of chlorine weighing 35 3 6, so that its molecule, therefore, must weigh 33 - 3G. But since the density of hydrochloric acid gas is ascertained by experiment to be only 18 18 as compared with that of hydrogen, and, according to Avogadro s hypothesis, equal volumes of hydrogen and hydrochloric acid gas contain equal numbers of molecules, it follows that the weight of the hydrogen molecule as compared with that of hydro chloric acid must be 2, or in other words, that the hydrogen molecule consists of two atoms. The chlorine molecule in like manner must consist of two atoms, each weighing 35 36, and in the formation of hydrochloric acid from hydrogen and chlorine two molecules of hydrochloric acid are produced from a molecule of hydrogen and a molecule of chlorine : in the one molecule half the hydrogen is displaced by chlorine, in the othsr half the chlorine is dis placed by hydrogen. It may be proved that the assumption is correct that the molecule of hydrochloric acid contains only a single atom of chlorine, weighing 35 36, and that it does not consist, for example, of two atoms of chlorine each weighing 1 7 68, by comparing the various volatile com pounds containing chlorine. la the first place their densities in the state of gas are determined, and a knowledge i-j thus obtained of the relative weights of their molecules as compared with that of the hydrogen molecule ; the percentage of chlorine they contain is then ascertained by careful analysis. The density referred to hydrogen as unity multiplied by 2 gives the molecular weight of the com pound ; and the percentage of chlorine being known, the amount contained in the quantity expressed by the molecular weight is ascertained by a simple calculation. For example, the density of sulphur chloride is found to be 57 - 36, and its molecular weight is therefore 57 36 x 2 or 114 72 ; it contains 61 64 per cent, of chlorine, so that in 11472 parts there are 70 72 of chlorine When the numbers thus deduced are compared it is seen that the lowest amongst them is 35 36, and that all the higher numbers are simple multiples of this ; 35 36 is accordingly adopted as the number which expresses the weight of the atom of chlorine relatively to that of the hydrogen atom. A number of volatile chlorine compounds are thus compared in the following table : Name of Compound. Molecular Weight. Weight of Chlorine. Hydrochloric acid Methyl chloride 36-36 50-33 35-36 35 36 Carbon oxychloride Mercuric chloride 98-65 270-52 35-36x2 35 36 x 2 Boron chloride.. 117-08 35 36 x 3 Phosphorus trichloride.. Carbon tetrachloride Silicon tetrachloride Aluminium chloride .... Chloriue. . 137-04 153-41 169-44 266-76 70-72 35-36x3 35-36x4 35-36x4 35-36x6 35 2Cx 2 In like manner, on comparing the various volatile com pounds which contain oxygen, it is found that the number 16 represents the least weight of oxygen contained in the molecular weight of any of its compounds ; 16 is therefore taken as the atomic weight of oxygen. In all cases in which it is possible to obtain volatile compounds, the atomic weights of elements may be deduced in this manner ; unfortunately, however, many of the elements do not furnish stable volatile compounds, so that hitherto the atomic weights of the following elements only have been ascertained by the application of Avogadro s hypothesis : Antimouy. lodiue. I Silicon. Arsenic. Lead. Sulphur. Bismuth. Mercury. Tantalum. Boron. Molybdenum. Tellurium Bromine. Niobium. I Tin. Carbon. Nitrogen. Titanium. Chlorine. Osmium. Tungsten. Chromium Oxygen. Vanadium. Fluorine. Phosphorus. Zinc. Hydrogen. Selenium. Zirconium. The determination of the density of bodies in the state of gas is thus an operation of fundamental importance. The precise manner in which the determination is effected is described in most of the text-books on chemistry. The methods ordinarily employed in the case of liquids and solids which by the application of heat can be converted into vapour or gas without undergoing decomposition are known respectively as Dumas s and Gay-Lussac s, a modifica tion of the latter method of great value has recently been introduced by Hofmann. By Dumas s method the weight of substance is ascertained which will furnish a certain volume of gas at a certain temperature and pressure ; by Gay-Lussac s method, however, and by Hofmann s modifica tion of it, the volume of gas is measured which is furnished by a given weight of the substance at a certain temperature and pressure. By either method we arrive finally at a knowledge of the weight (w) of a certain volume (?;) of the gas at a temperature t and pressure p and its density (D) referred to hydrogen is then found by dividing the weight w by the weight (iv) of an equal volume of hydrogen at the same temperature t and pressure p w/ Or the density referred to air may be calcu.ated in a similar manner, and then converted into the density referred to hydrogen by multiplication by 14 43, the number which expresses the density of air referred to hydrogen. Both methods require that the substance be heated to the tem perature at which its vapour most closely approximates to the laws of Boyle and Charles, which is readily ascertained by experiment ; this temperature, however, is often very considerably above the boiling point of the substance, and acetic acid may be cited as an illustration of this. Thus, although this acid boils at 119 C., its vapour does not exhibit the required density until it is heated to 250 C., as will be evident from the following table the theoretical vapour density of acetic acid vapour referred to hydrogen being about 30 : Temperature ....... 125 130 140 160 190 250 300 Vapour Density... 46-1 45 -0 41 8 357 33 1 30 &quot;01 30 01 Owing to unavoidable experimental errors, and, in many cases, probably to the circumstance that the vapours of solid and liquid bodies are very imperfect gases at temperatures not much above their boiling points, the determination of the vapour density of a substance does not, as a rule, furnish a result of more than approximate accuracy, the result being the more accurate, however, the more rarefied the vapour and the higher the temperature at which the