INTRODUCTION. 



In the autumn of 1877 the writer began collecting data 

 relative to the determinations of atomic weights, with the 

 purpose of preparing a complete resume of the entire sub- 

 ject, and of recalculating all the estimations. The work 

 was fairly under way, the material was collected and partly 

 discussed, when I received from the Smithsonian Institu- 

 tion a manuscript by Professor George F. Becker, entitled 

 " Atomic Weight Determinations : a Digest of the Investi- 

 gations Published since 1814." This manuscript, which has 

 lately been issued as Part IV of the " Constants of Nature," 

 covered much of the ground contemplated in my own 

 undertaking. It brought together all the evidence, pre- 

 senting it clearly and thoroughly in compact form ; in 

 short, that portion of the task could not well be improved 

 upon. Accordingly, I decided to limit my own labors to a 

 critical recalculation of the data ; to combine all the figures 

 upon a common mathematical basis, and to omit everything 

 which could as well be found in Professor Becker's " Digest." 



At the very beginning of my work certain Cjuestions con- 

 fronted me. Should I treat the investigations of different 

 individuals separately, or should I combine similar data to- 

 gether in a manner irrespective of persons ? For example, 

 ought I, in estimating the atomic weight of silver, to take 

 Stas' work by itself, Marignac's work by itself, and so on, 

 and then average the results together; or should I rather 

 combine all series of figures relating to the composition of 

 potassium chlorate into one mean value, and all the data 

 concerning the composition of silver chloride into another 

 mean, and, finally, compute from such general means the 

 constant sought to be established ? The latter plan was 

 finally adopted ; in fact, it was rendered necessary by the 

 method of least squares, which method was alone adequate 

 to supply me with good processes for calculation. 



(VII) 



