ATOMIC WEIGHTS 7 



The data of Eichards and his collaborators for the chlorides of tliese 

 metals give a ratio between silver and chlorine in agreement with the 

 measurements by Stas, and lower than that which Eichards and Wells 

 have established. If, therefore, the work of Stas is in error, the same 

 error inheres in the atomic weights of the three metals above mentioned, 

 and the latter, even if the uncertainty is small, ought to be revised. The 

 sharp concordance found when the atomic weights were computed with 

 Stas' figures is either illusive, or else the modern data for silver and 

 chlorine are wrong. The first of these alternatives is the more probable. 

 In spite of the discordance now evident, the determinations for caesium, 

 barium and magnesium are by far the best we have, and their uncer- 

 tainties need not be regarded as serious. 



In Meyer and Seubert's recalculation, weights are assigned in quite a 

 novel manner. In each series of experiments the maximum and mini- 

 mum results are given, but instead of the mean there is a value deduced 

 from the sum of the weighings — that is, each experiment is weighted 

 proportionally to the mass of the material handled in it. For this 

 method I am unable to find any coiuplete justification. Of course, the 

 errors due to the operations of weighing become proportionally smaller 

 as the quantity of material increases, but these errors, with modern 

 apparatus, are relatively unimportant. The real errors in atomic weight 

 determinations are much larger than these, and due to different causes. 

 Hence an experiment upon ten grammes of material may be a little better 

 than one made upon five grammes, but it is by no means necessarily 

 twice as good. The ordinary mean of a series of observations, with its 

 measure of concordance, the probable error, is a better value than one 

 obtained in the manner just described. If only errors of weighing were 

 to be considered, Meyer and Seubert's summation method would be 

 valid, but in the presence of other and greater errors it seems to have 

 but little real pertinency to the problem at hand. 



In addition to the usual periodicals, the following works have been 

 freely used by me in the preparation of this volume: 



Berzelius, J. J. Lehrbuch der Chemie. 5 Auflage. Drifter Band. 

 SS. 1147-1231. 1845. 



Van Geuns, W. A. J. Proeve eener G-eschiedenis van de ^^quivalent- 

 getallen der Scheikundige Grondstoffen en van hare Soortelijke 

 Gewigten in Gasvorm, voornamelijk in Betrekking tot de vier 

 Grondstoffen der Bewerktuigde Natuur. Amsterdam, 1853. 



Mulder, E. Historisch-Kritisch Overzigt van de Bepalingen der ^Equiv- 

 alent-Gewigten van 13 Eenvoudige Ligchamen. Utrecht, 1853. 



Mulder, L. Historisch-Kritisch Overzigt van de Bepalingen der ^quiv- 

 alent-Gewigten van 24 Metalen. Utrecht, 1853. 



