fyxmll Winivmii^ ^iiuxi^ THE GIFT OF C This result, namely, 132.867 parts of the chloride from 100,000 of silver, has a " probable error," computed according to the method of least squares of about 0.0005. This so-called probable error of course *T'he hydrochloric acid used in these experiments was three times succes- sivdiy treated with small amounts of permanganate and boiled to 'diminate bro- mide, and each time idistilled. 66 ATOMIC WEIGHTS OF SODIUM AND CHLORINE. gives no clue as to possible constant errors involved. A better idea of the chemical trustworthiness of the result is got by comparing the data furnished by the two different methods of synthesis. The averages of the seven determinations by the method of filtration is 132.8666, while that of the three by the method which involves no transference of material (83, 84, 85) is 132.8673. These are both within the range from the mean 132.8668 indicated by the " probable errors" of the respective averages, and hence may be considered as identical. If the comparison be made according to the environment of the silver during fusion, a similar result is obtained. The silver fused in hydrogen gives an average of 132.8668, while that fused in a vacuum gives 132.8668. Again this is essential identity. In view of these facts, the fact that all probable errors tend to lower the result, and the multitude of precautions which eliminated error from these figures, it is not unsafe to conclude that locooo parts of the purest silver really yields as much as 132.867 parts of argentic chloride. This conclusion confirms the comparison of the silver and argentic chloride indicated by the work on sodium, and shows that no great occlusion of sodic nitrate could have taken place there. It furnishes, moreover, a means of determining by comparison the purity of any other specimen which has been used in the quantitative synthesis of argentic chloride by either of the foregoing methods. For example, it shows that the silver used in the work on sodium — silver which yielded 132.862* parts of chloride — must have contained j3^ = ^5oo=' 0.0037 psr cent of impurity. *See Syntheses 65, 66, and 70 given in the table on page 61. The silver used in these syntheses was exactly similar to some of that used in the work on sodium. In the same way, if Stas's method of synthesis is considered as com- parable with ours, his silver must have contained (132867 — 132848) -=- 132867 = o.ois per cent of impurity. It is doubtful, however, if the imethods of synthesis are strictly comparable. EFFECT OF NEW ATOMIC WEIGHTS. 67 THE NEW ATOMiC WEIGHTS OF SODIUM AND CHLORINE AND THEIR EFFECT ON OTHER ATOMIC WEIGHTS. From the preceding description of the syntheses of argentic chloride, it is clear that 100.000 grams of the purest metal yields 132.867 grams of chloride. If then the atomic weight of silver is taken as 107.920* (a convenient value to assume for preliminary calculation), the molecular weight of argentic chloride becomes by simple proportion 143.393, and by difference the atomic weight of chlorine becomes 35.470.! This value is about 0.05 per cent greater than the value announced by Stas ; and the change produces a serious effect on a number of atomic weights. It is, of course, impossible to be perfectly certain of the accuracy of the new value, because it may contain concealed within it some entirely unsuspected source of error, but at least it is free from the real mistakes in the earlier work. It was reasonably certain that at least ten more syntheses would be needed to affect the value one unit in the third decimal place, if the further results varied no more widely than those recorded above in the final series. Hence it seems unlikely that chlorine is above 35.471 or below 35.469 if silver is taken as 107.920. What, now, is the effect of this new value upon the atomic weight of sodium? In the table given upon page 44 it is shown that 100.000 grams of argentic chloride could be obtained from 40.780 grams of sodic chloride. If now argentic chloride is taken as 143.390, sodic chloride becomes 58.474, and by subtracting the new value for chlorine, 23.004 is obtained as the new value for sodium. This is not, however, the only value for sodium which may be calculated from our results. On page 52 has been given a table show- ing the results of ten experiments on the comparison of sodic chloride with silver direct, and on page 56 the results of two more. These deter- minations were made before we had discovered the best method of obtaining pure silver ; but the trace of impurity is allowed for in those tables, and it is safe to conclude that 100.000 parts of the purest silver are equivalent to 54.185 parts of sodic chloride. If, then, silver is taken as 107.920 and chlorine as 35.470, sodium becomes 23.007. Thus two entirely independent values, calculated from two entirely- separate series of experiments, yield respectively the values 23.004 and 23.007 for the atomic weight of sodium. Of these two, the latter is *F. W. Clarke has for a long time accepted this value, and it is now assumed because it is probably nearer the true value than 107.93. fin tihis connection it should be pointed out that Leduc called attention to the fact .that if Stas's silver really contained as much oxygen as Dumas said it must, chlorine -would become 35.47. Compt. Rend. (1901). 68 ATOMIC WEIGHTS OF SODIUM AND CHLORINE. somewiiat the more trustworthy, because, as has been said, the occlu- sion of sodic nitrate by the precipitate does not afifect it ; hence the mean may be taken as 23.006. This value is probably not more in error than two units in the third decimal place, if silver is 107.920, although of course the same remark applies to it that was made concern- ing the possible error of the value for chlorine. If silver is taken as 107.930, sodium becomes 23.008, and chlorine 35.473. The new values are, then, as follows : (Ag=l07.920). (Ag=l07.930). Atomic weight of sodium 23.006 23.008 Atomic weight of chlorine 35.470 35.473 The new value for sodium is nearly 0.2 per cent less than that found by Stas — a percentage error much greater even than that made by him in the case of iodine. It will be noted that the likely error of the new result is only about a twentieth of the difference between the new result and that of Stas. Some will attach significance to the fact that both the new values bring the elements nearer to the demands of Prout's hypothesis. These new values affect greatly the figures in the second decimal place of all other atomic weights depending directly or indirectly upon chlorine, sodium, or silver. The number of elements thus affected is so great and the relations so complicated as to render imperative a re- ca,lculation of all atomic weights. Nitrogen, as computed from am- monic chloride, will be especially affected, the new value approaching more nearly that required by Avogadro's rule than the old. Probably, however, it will be best to delay this systematic recalculation until a few other new data shall have been obtained — in particular new analyses of potassic chloride, argentic chlorate, the bromides, sulphides and sulphates, and similar important compounds. Some of these are already under way at Harvard, and others will be undertaken at once. In conclusion, it is a pleasure to acknowledge our great indebted- ness to the Carnegie Institution of Washington, without whose liberal support the present investigation could not have attained its present thoroughness or precision. SUMMARY. 69 SUMMARY. The investigation consisted of a very careful quantitative study of three ratios — namely, AgCl : NaCl, Ag : NaCl, and Ag : AgCl. The effort was made to test every operation involved in the execution of the experiments. In the course of the work, the following points were developed : 1. Sodic chloride, obtained from many sources and purified in many ways, always gave the same equivalent weight. 2 . Fusion in vacuum of salt already fused in air caused no change in this equivalent weight. 3. Argentic chloride precipitated from aqueous solutions always occludes traces of other substances present, and those traces can not always be eliminated. Very dilute solutions must hence be used in precipitation. 4. The conditions governing the occlusion and release of these impurities were minutely studied, and it was shown that Stas's method of dropping solid salt into a silver solution causes occlusion of salt. Many considerations important in all precise chemical work are dis- cussed. 5. A careful study of the solubility of argentic chloride was made, and the precautions necessary in using the nephelometer in the estima- tion of traces of chloride and silver were ascertained. 6. Fused argentic chloride probably contains traces of dissolved air, but not enough to affect essentially its weight, since subsequent fusion in vacuum caused no appreciable loss of weight. 7. The most difficult question in the purification of silver was found to be the elimination of the inclosed mother liquor without introducing other impurities. Fusion on pure lime, first in pure hydrogen and then in a vacuum, is the safest method. Stas's silver must have contained at least as much oxygen as Dumas claimed. 8. In ten experiments, 44.5274 grams of sodic chloride yielded 109.1897 grams of argentic chloride. 9. In twelve other experiments, entirely distinct from these, 49.5007 grams of sodic chloride were found to be equivalent to 91 .3543 grams of the purest silver. 10. In ten other experiments, again entirely distinct from the preceding, 82.6689 grams of the purest silver yielded 109.8395 grams of argentic chloride. Two very different methods of synthesis were used in this series, and the silver came from various sources, these variations being without effect on the result. 70 ATOMIC WEIGHTS OF SODIUM AND CHLORINE. 11. If the atomic weight of silver is assumed to be 107.920, sodium is found from the above results to have an atomic weight of 23.006 and chlorine an atomic weight of 35.470. 12. Many other atomic weights are affected, in their second deci- mal places, by these changes. In particular, certain slight anomalies previously noticed in Harvard work are explained by them, and the atomic weight of nitrogen computed from ammonia is brought nearer to the value required by Avogadro's rule. Other anomalies appear in other places, however, and it is clear that many new atomic-weight investigations must be instituted to explain them, with due attention to hitherto unheeded dangers, especially of occlusion.