He showed also, that when the two gases after combining remained in the gaseous state, the diminution of volume was either 0, or ⅓, or ½. The constancy of these proportions left no doubt that the combinations of all gaseous bodies were definite. The theory of Dalton applied to them with great facility. We have only to consider a volume of gas to represent an atom, and then we see that in gases one atom of one gas combines either with one, two, or three atoms of another gas, and never with more. There is, indeed, a difficulty occasioned by the way in which we view the composition of water. If water be composed of one atom of oxygen and one atom of hydrogen, then it follows that a volume of oxygen contains twice as many atoms as a volume of hydrogen. Consequently, if a volume of hydrogen gas represent an atom, half a volume of oxygen gas must represent an atom. Dr. Prout soon after showed that there is an intimate connexion between the atomic weight of a gas and its specific gravity. This indeed is obvious at once. I afterwards showed that the specific gravity of a gas is either equal to its atomic weight multiplied by 1·111[.1] (the specific gravity of oxygen gas), or by 0·555[.5] (half the specific gravity of oxygen gas), or by O·277[.7] (1-4th of the specific gravity of oxygen gas), these differences depending upon the relative condensation which the gases undergo when their elements unite. The following table exhibits the atoms and specific gravity of these three sets of gases: I. Sp. Gr. = Atomic Weight × 1·1111 Atomic Sp. weight. gravity. Oxygen gas 1 1·1111 Fluosilicic acid 3·25 3·6111 II. Sp. Gr. = Atomic Weight × 0·555[.5]. Atomic weight. Sp. gravity. Hydrogen 0·125 0·069[.4] Azotic 1·75 0·072[.2] Chlorine 4·5 2·5 Carbon vapour 0·75 0·416[.6] Phosphorus vapour 2 1·111[.1] Sulphur vapour 2 1·111[.1] Tellurium vapour 4 2·222[.2] Arsenic vapour 4·75 2·638[.8] Selenium vapour 5 2·777[.7] Bromine vapour 10 5·555[.5] Iodine vapour 15·75 8·75 Steam 1·125 0·625 Carbonic oxide gas 1·75 0·972[.2] Carbonic acid 2·75 1·527[.7] Protoxide of azote 2·75 1·527[.7] Nitric acid vapour 6·75 3·75 Sulphurous acid 4 2.222[.2] Sulphuric acid vapour 5 2·777[.7] Cyanogen 3·25 1·805[.5] Fluoboric acid 4·25 2·361[.1] Bisulphuret of carbon 4·75 2·638[.8] Chloro-carbonic acid 6·25 3·472[.2] III. Sp. Gr. = Atomic Weight × 0·277[.7].