composed by nitric acid, and the silver thus taken into solution was titrated with standard sodium chloride. In three others the tungstate was treated directly with common salt, and the residual silver chloride collected and weighed. Here again, on account of some complexity in Zettnow's figures, I am compelled to reduce his data to a common standard. To 100 parts of AgCl the following quantities of Ag2 WO, correspond: By First Method. 161.665 161.603 Mean, 161.634, ± .021 By Second Method. 161.687 161.651 161.613 Mean, 161.650, ± .014 General mean from both series, 161.645, .012 Finally, we have two analyses by Roscoe of tungsten hexchloride, published in the same paper with his results upon the trioxide. In one experiment the chlorine was determined as AgCl; in the other the chloride was reduced by hydrogen, and the residual tungsten estimated. By bringing both results into one form of expression we have for the percentage of chlorine in WCl ̧:* 6 53.588 53.632 Mean, 53.610, ± .015 We have now five ratios from which to calculate the atomic weight of tungsten: (1.) Percentage of W in WO3, 79.3215, ±.00085 (2.) Percentage of H2O in BaO.4WO3.9H2O, 13.0368, ± .0060 *The actual figures are as follows: 19.5700 grm. WCl gave 42.4127 grm. AgCl. (3.) Am,Fe(SO4)2.6H2O: FeWO̟ : : .0365457, ± .0000012: .0283549, ± .0000115 4 (4.) AgCl: Ag2WO:: 100: 161.645, ± .012 From these we get five values for tungsten, as follows: It is not the purpose of the present investigation to examine at all systematically such questions as are involved in the discussion whether the atomic weight of uranium is 120 or 240. For convenience we may use the formulæ based upon the smaller number, and, if eventually the larger value proves to be correct, it will be easy to double the figures which we obtain. Suffice it to say here, that the specific heat of the green oxide, according to Donath,* agrees best with the formula U ̧O̟ and the lower atomic weight. On the other hand, the value 240 fits best into such schemes as that given by Mendelejeff in his paper on the periodic law. An accurate determination of the specific heat of the metal itself is much needed, for the material with which Regnault worked was of uncertain quality; furthermore, the vapor density of some volatile uranium compounds ought to be ascertained.† Until some such data have been rigidly 3 *Ber. d. Deutsch. Chem. Gesell., 12, 742. 1879. The value of 240 for uranium is strongly sustained by the recent experiments of Zimmermann upon the vapor density of the tetrachlorid and tetrabromid. For UBr, the vapor density is 19.46, while theory (U = 240) requires 19.36. For UCl the v. d. 13.33 was found. Gesell., 14, s. 1934. 1881.) Theory, 13.21. (Ber. der Deutsch. Chem. established the controversy over the two rival values can hardly be satisfactorily settled. The earlier attempts to determine the atomic weight of uranium were all vitiated by the erroneous supposition that the uranous oxide was really the metal. The supposition, of course, does not affect the weighings and analytical data which were obtained, although these, from their discordance with each other and with later and better results, have now only a historical value. 3 For present purposes the determinations made by Berzelius,* by Arfvedson,† and by Marchand,‡ may be left quite out of account. Berzelius employed various methods, while the others relied upon estimating the percentage of oxygen lost upon the reduction of U,О, to UO. Rammelsberg's || results also, although very suggestive, need no full discussion. He analyzed the green chloride, UCl,; effected the synthesis of uranyl sulphate from uranous oxide; determined the amount of residue left upon the ignition of the sodio and bario-uranic acetates; estimated the quantity of magnesium uranate formed from a known weight of UO, and attempted also to fix the ratio between the green and the black oxides. His figures vary so widely that they could count for little in the establishing of any general mean; and, moreover, they lead to estimates of the atomic weight which are mostly below the true value. For instance, twelve lots of UO, from several different sources were reduced to UO by heating in hydrogen. The percentages of loss varied from 3.83 to 4.67, the mean being 4.121. These figures give values for the atomic weight of uranium ranging from 92.66 to 117.65, or, in mean, 107.50. Such discordance is due partly to impurity in some of the material studied, and illustrates the difficulties inherent in the problem to be solved. Some of the uranoso-uranic oxide was prepared by * Schweigg. Journ., 22, 336. 1818. Poggend. Annal., 1, 359. 1825. || Poggend. Annal., 55, 318, 1842; 56, 125, 1842; 59, 9, 1843; 66, 91, 1845. Journ. für Prakt. Chem., 29, 324. calcining the oxalate, and retained an admixture of carbon. Many such points were worked up by Rammelsberg with much care, so that his papers should be scrupulously studied by any chemist who contemplates a redetermination of the atomic weight of uranium. In 1841 and 1842 Peligot published certain papers* showing that the atomic weight of uranium must be somewhere near 120. A few years later the same chemist published fuller data concerning the constant in question, but in the time intervening between his earlier and his final researches other determinations were made by Ebelmen and by Wertheim. These investigations we may properly discuss in chronological order. For present purposes the early work of Peligot may be dismissed as only preliminary in character. It showed that what had been previously regarded as metallic uranium was in reality an oxide, but gave figures for the atomic weight of the metal which were merely approximations. Ebelmen's determinations of the atomic weight of uranium were based upon analyses of uranic oxalate. This salt was dried at 100°, and then, in weighed amount, ignited in hydrogen. The residual uranous oxide was weighed, and in some cases converted into U3O, by heating in oxygen. The following weights are reduced to a vacuum standard: 10.1644 grm. oxalate gave 7.2939 grm. UO. 12.9985 66 4 Gain on oxidation, .3685 9.3312 Reducing these figures to percentages, we may present the results in two columns. Column A gives the percentages of UO in the oxalate, while B represents the amount of U30, formed from 100 parts of UO: 4 * Compt. Rend., 12, 735. 1841. Journ. für Prakt. Chem., 27, 385. Ann. Chim. Phys., (3,) 55. 1842. Wertheim's experiments were even simpler in character than those of Ebelmen. Sodio-uranic acetate, carefully dried at 200°, was ignited, leaving the following percentages of sodium uranate: The final results of Peligot'st investigations appeared in 1846. Both the oxalate and the acetate of uranium were studied and subjected to combustion analysis. The oxalate was scrupulously purified by repeated crystallizations, and thirteen analyses, representing different fractions, were made. Seven of these gave imperfect results, due to incomplete purification of the material; six only, from the later crystallizations, need to be considered. In these the uranium * Journ. für Prakt. Chem., 29, 209. 1843. † Compt. Rend., 22, 487. |