2 5 studied two compounds of vanadium; namely, the pentoxide, V2O, and the oxychloride, VOCI,. The pentoxide, absolutely pure, was reduced to V2O, by heating in hydrogen, with the following results: 7.7397 grm. V2O, gave 6.3827 grm. V2O3. 2 3 17.533 per cent. of loss. 5.4296 grm. V2O, reoxidized, gave 6.5814 grm. V2O. 17.501 per cent. difference. Upon the oxychloride, VOCl3, two series of experiments were made, one volumetric, the other gravimetric. In the volumetric series the compound was titrated with solutions containing known weights of silver, which had been purified according to the methods recommended by Stas. Roscoe publishes his weighings, and gives percentages deduced from them; his figures, reduced to a common standard, make the quantities of VOCI, given in the third column proportional to 100 parts of silver. He was assisted by two analysts: 3 The gravimetric series, of course, fixes the ratio between VOCI, and AgCl. If we put the latter at 100 parts, the proportion of VOC, comes out as given in the third column: These two series give us two values for the molecular weight of VOCI,: Combining the two values for V we get the following For the determination of the atomic weight of arsenic two compounds have been studied; the chloride and the trioxide. The bromide may also be considered, since it was analyzed by Wallace in order to establish the atomic weight of bromine. His series, in the light of more recent knowledge, may properly be inverted, and applied to the determination of arsenic. In 1826, Berzelius* heated arsenic trioxide with sulphur * Poggend. Annal., 8, 1. 2 in such a way that only SO, could escape. 2.203 grammes of As,O,, thus treated, gave a loss of 1.069 of SO,. Hence As = 74.840. This is a close estimation; but, being drawn from a single experiment, has so little weight that it need not be included in our final general mean. = = In 1845 Pelouze* applied his method of titration with known quantities of pure silver to the analysis of the trichloride of arsenic, AsCl,. Using the old Berzelian atomic weights, and putting Ag 1349.01, and Cl 443.2, he found in three experiments for As the values 937.9, 937.1, and 937.4. Hence 100 parts of silver balance the following quantities of AsCl ̧: 56.029 56.009 56.016 Mean, 56.018, ± .004 Later, the same method was employed by Dumas,† whose weighings, reduced to the foregoing standard, give the following results: The two series of Pelouze and Dumas, combined, give a general mean of 56.014, .0035, as the amount of AsCl, equivalent to 100 parts of silver. Hence As 74.829, ± .048, a value closely agreeing with that deduced from the single experiment of Berzelius. = The same process of titration with silver was applied by Wallace to the analysis of arsenic tribromide, AsBr. This compound was repeatedly distilled to ensure purity, and was well crystallized. His weighings show that the quanti * Compt. Rend., 20, 1047. Ann. Chim. Phys., (3,) 55, 174. 1859. ties of bromide given in the third column are proportional to 100 parts of silver: Hence As 74.046, = .058. Why this value should be so much lower than that from the chloride is unexplained. The volumetric work done by Kessler,* for the purpose of establishing the atomic weights of chromium and of arsenic, has already been described in the chromium chapter. In that investigation the amount of potassium dichromate required to oxidize 100 parts of As, O, to As, 0, was determined, and compared with the quantity of potassium chlorate necessary to produce the same effect. From the molecular weight of KCIO,, that of K2Cr,O, was then calculable. 2 2 3 From the same figures, the molecular weights of KCIO, and of K, Cr2O, being both known, that of As, O, may be easily determined. The quantities of the other compounds proportional to 100 parts of As, O, are as follows: 3 * Poggend. Annal., 95, 204. 1855. Also 113, 134. 1861. Another series with the bichromate gave the following figures: 99.08 99.06 99.10 98.97 98.97 Mean, 99.036, ± .019 Mean of previous series, 99.045, ± .028 General mean, 99.039, ±.016 Other defective series are given to illustrate the partial oxidation of the As, O, by action of air. The foregoing figures give us two distinct values for the molecular weight of As,O,. In calculating from the bichromate results the value for chromium deduced from Siewert's determinations will be used, viz., Cr 52.009, ± .025. = The general mean for As comes out as follows: After some earlier, unsatisfactory determinations, Berzelius,* in 1826, published his final estimation of the atomic weight of antimony. He oxidized the metal by means of nitric acid, and found that 100 parts of antimony gave 124.8 of Sb2O4. Hence, if 0 = Hence, if = 16, Sb 129.03. The * Poggend. Annal., 8, 1. = |