Table showing the Density of several Compound Gases and Vapours. *Here it is assumed that I atom of carbon yields 2 volumes of vapour. Many chemists consider that it yields only I volume, but no data exist for proving either hypothesis. Sulphur, in accordance with Bineau's researches upon it at high temperatures, is assumed to have an atomic volume equal to that of oxygen; and selenium and tellu rium have been shown by Deville and Troost to be analogous to it. Table showing the Density of several Compound Gases and Vapours. *The vapour volume of one atom of each of the different metals is assumed to correspond with that of an atom of carbon, and consequently to be double that of the atom of hydrogen; and the same assumption is made in the case of silicon and of boron. ANOMALOUS VAPOUR DENSITIES. 955 In the foregoing table all compounds into the composition of which carbon and hydrogen both enter are represented by a molecular weight equal to 2 volumes of vapour (H=1 being taken as unity); and in the majority of other cases the formulæ given are also 2-volume formulæ. This, indeed, is the normal volume of bodies in the aëriform state. Exceptions, however, occur in the case of the salts of ammonia; the hydrochlorate, hydrobromate, hydrocyanate, and the hydrosulphate of the sulphide (H,NHS) furnish 4 volumes of vapour. Cannizzaro and Kopp have each endeavoured to explain this anomaly on the supposition that at the high temperature necessary to volatilize these bodies decomposition takes place, and the salt is resolved into its two constituents, ammonia and the gaseous acid; and as the temperature falls they enter again into combination.* This view is ingenious, and it is also applicable to a case, such as that of the hydrosulphate of ammonia (H ̧N),H,S, which yields a 6-volume vapour such as would be produced by the separation of the compound into its constituents, ammonia (4 vols.) and sulphuretted hydrogen (2 vols.). Similar explanations have been suggested in the case of hydriodate and hydrobromate of phosphuretted hydrogen, the molecule of each of which yields 4 volumes of vapour. Anhydrous carbamate of ammonium H ̧NH,NEO,, according to the observation of Bineau, yields 6 volumes of vapour; and this would be the result if it broke up into 2 H,N=4 vols., and EO, 2 vols. In like manner Regnault observed that the compound Cl2, obtained by acting with chlorine upon methylic ether, yields 4 vols. of vapour, which may be explained on the supposition that when heated it splits into phosgene EOC, (2 vols.), and carbonic tetrachloride EC1, also 2 vols. Phosphoric chloride (PCI) is also a body which yields 4 vols. of vapour, and which, at high temperatures, it has been suggested may be resolved into phosphorous chloride, PCl。 (2 vols.), and free chlorine (Cl2), also 2 vols.; an hypothesis which has been directly confirmed by the experiments of Deville upon the chloride, and upon phosphoric bromide (PBr), which exhibits a similar anomaly. There are, however, cases in which such an explanation is inadmissible. Chlorous anhydride (Cl) is a 3-volume gas at ordinary temperatures; but it is clear that it is a true combination, * Deville, however, finds that muriate of ammonia may be heated to 1900° F., and on cooling is still found in combination; whilst ammonia, if in a free state, would at that temperature be decomposed into its constituent gases. 956 ATOMIC VOLUMES OF SOLIDS-SIMPLE BODIES. and not a mixture of chlorine and oxygen. Nitric oxide, N.,, is a 4-volume gas, whilst nitrous oxide, N,O, exhibits the normal condensation of 2 volumes. 8 Anhydrous orcin (E,H,,), according to the observations of Dumas (p. 695), appeared to have an anomalous vapour density, but the recent experiments of V. Luynes show that it is really normal, the molecule yielding 2 volumes of vapour. Some remarkable irregularities have been observed in the volume occupied by the vapours of many volatile compounds. Many of these vapours at a low temperature have a density much greater than that which they possess when more strongly heated. The vapours of the volatile acids, such as the formic and the acetic acids in particular, exhibit this anomaly in a marked degree (1274). (1731) Atomic Volumes of Solids.—1. Simple Bodies.—It has been supposed that if the atoms of all the elementary bodies were of the same size, the specific gravities of the bodies in their solid form would be in the same proportion as their atomic weights. In such a case, however, either the particles composing the solid must be in actual contact, or the intervals between the particles must be equal. Dumas showed many years ago that the specific gravity of certain isomorphous metals was nearly in the direct ratio of their atomic weights. Since then both Schröder and Kopp have pointed out a number of remarkable relations between the densities of different bodies and their atomic weights. Kopp has shown that many of the elementary bodies may be arranged in groups, each group consisting of members in which the solid atomic volume is identical. If such weights of the different elementary bodies as represent the atomic weights of each be compared together, the bulk occupied by each body will be such as is indicated in the column headed atomic volume in the table on the opposite page. The atomic volume (or specific volume) of any substance, simple or compound, may be calculated by dividing its atomic weight by its specific gravity. Thus, if d=the density or specific gravity, q=the atomic weight, and e=the atomic volume, 2=v. A simple mode of determining experimentally the atomic volume of a body was employed by Playfair and Joule. Their apparatus or volumenometer consists of a globular flask provided with a long narrow neck, about twelve inches in length, graduated from below upwards, to indicate grains of water. The flask is provided with a tubulure (accurately fitted with a ground stopper) for the admission of the solid body for experiment. When the |