purified either by precipitation as chloride and reduction with invert sugar, or by electrolysis, or by precipitation with ammonium formiate. The metal was then fused before a blowpipe upon a crucible of the purest lime, and the buttons were thoroughly cleansed with nitric acid. No further purification was considered necessary since the weight of the metal was of no consequence. After the silver bromide had been washed by decantation with water, in some cases it was collected in a Gooch crucible in which a disk of filter paper was employed instead of asbestos, and after drying at 100° it was carefully separated from the filter paper. In other cases the precipitate was transferred to a platinum dish, and was drained with a platinum reverse filter 21 with a disk of filter paper. In still others a platinum Gooch crucible with small holes was found to be sufficiently effective as a filtering medium without the use of either asbestos or filter paper. Before being weighed the silver bromide was fused in a current of air saturated with bromine in a weighed quartz crucible. The air was purified by passing successively over beads moistened with silver nitrate solution, over sodium carbonate, and finally over concentrated sulphuric acid which had been heated to its boiling-point with a small quantity of recrystallized potassium dichromate to eliminate volatile and oxidizable impurities. The air was then passed through dry bromine in a small bulb. This apparatus was constructed entirely of glass with ground joints. The tube which conducted the gases into the crucible passed through a Rose crucible cover of glazed porcelain in all experiments except Analyses 28 to 31, in which a quartz cover was employed. The quartz crucibles were always contained in large porcelain crucibles while being heated. They remained almost absolutely constant in weight during the experiments. The bromine was in each case a portion of the sample from which the silver bromide had been made. Next the bromide was heated barely to fusion in a slow current of chlorine, generated by the action of hydrochloric acid upon manganese dioxide, and dried by means of concentrated sulphuric acid. The apparatus for this purpose also was constructed wholly of glass. When the bromine was apparently completely displaced, the silver chloride was heated in the air for a few minutes to expel dissolved chlorine, and then was cooled and weighed. A repetition of the heating in chlorine seldom affected the weight of the salt more than a few hun 21 Cooke, These Proceedings, 12, 121. dredths of a milligram, although occasionally a third heating was necessary to effect this result. That no loss of silver chloride by volatilization took place is certain for two reasons. In the first place the cover of the crucible and the delivery tube for the bromine when rinsed with ammonia and the solution treated with a slight excess of hydrochloric acid gave no visible opalescence in the nephelometer. In the second place the weight of the chloride became constant without difficulty. It has already been shown that silver chloride which has been fused in chlorine, if subsequently heated in air, retains no excess of chlorine.22 The following vacuum corrections were applied: silver bromide, +0.000041; silver chloride + 0.000071.23 The atomic weight of chlorine referred to silver 107.930 is assumed to be 35.473. Aside from the close agreement of all the results of Series I, the fact is to be emphasized that of the last seven analyses, which were consecutive, only two differ from the average of the series, 79.953, by as much as one one thousandth of a unit. Furthermore, there is no evidence of any dissimilarity in the different preparations of bromine. Material which has received only two distillations from a bromide gives values no lower than bromine which has been thus treated four times. The various specimens of silver also show no difference in purity. In the case of Series II, the extreme variation of the results is only four thousandths of a unit, and only one of the thirteen experiments yielded a value which differs from the average by more than one one thousandth of a unit. Finally, the difference between the averages of Series I and II is only seven ten thousandths of a unit. It is extremely unlikely that constant errors could have affected both series equally, so that this striking agreement is strong proof that both series are free from such errors. It has already been pointed out that the average of Stas's syntheses, 79.954, probably represents with considerable accuracy the atomic weight of bromine, and that certainly his determinations are more accurate than those of later experimenters. His syntheses are few in number, however, and differ among themselves by several thousandths of a unit, so that they do not define within this amount the constant in question. Their average, however, confirms the value 22 Baxter, These Proceedings, 40, 432 (1904); Richards and Wells, Publications of the Carnegie Institution, No. 28, page 59. 23 Richards and Stull have found the density of fused silver chloride to be 5.56. obtained in this paper. From all the experiments here described the number 79.953 seems to be the most probable value for the atomic weight of bromine. In conclusion, attention may be called to the fact that a diminution in the atomic weight of bromine raises slightly all atomic weights resulting from the analysis of metallic bromides by precipitation with silver. I am deeply indebted to the Carnegie Institution of Washington and to the Cyrus M. Warren Fund for Research in Harvard University for assistance in pursuing this investigation. CHEMICAL LABORATORY OF HARVARD COLLEGE, Proceedings of the American Academy of Arts and Sciences. VOL. XLII. No. 12.- OCTOBER, 1906. CONTRIBUTIONS FROM THE ZOÖLOGICAL LABORATORY OF THE MUSEUM OF COMPARATIVE ZOÖLOGY AT HARVARD COLLEGE, E. L. MARK, DIRECTOR. - No. 184. THE OPTIC CHIASMA OF TELEOSTS: A STUDY OF INHERITANCE. BY A. P. LARRABEE. |