30 PROCEEDINGS OF THE AMERICAN ACADEMY 



increase the apparent atomic weight, and, so fiir as our knowledge goes, 

 it would seem impossible that the value obtained in 13 D.,for example, 

 should be too low. Mot-eover, the black residue is not always wholly 

 carbon, and at times contains some antimony compound. Any siliceous 

 or other insoluble material which accident had introduced into the 

 analysis would of course be eliminated as a part of the residue ; and the 

 same would be true of all forms of organic matter, as well as tartaric 

 acid, which the precipitate might absorb from the solutions in which it 

 formed, or from the water by which it was washed.* It may be 

 interesting, in this connection, to add a few determinations of the 

 relative amount of combustible and incombustible matter in a few of 

 these residues : — 



No. 1. Weight of residue 0.0067 grammes 



„ after ignition 0.0032 „ 



Combustible 0.0035 „ 



No. 2. Weight of residue 0.0078 „ 



„ after ignition 0.0013 „ 



Combustible portion 0.0065 „ 



No. 3. Weight of residue 0.0064 „ 



„ after ignition 0.0020 „ 



Combustible portion 0.0044 „ 



Although this discussion of the causes of error was essential to the 

 refinement of the process, it must not be inferred that the magnitude 

 of these errors was proportional to the attention they have necessarily 

 received, or that they are important except with reference to the accu- 

 racy required in the determination of an atomic weight. Except in those 

 cases where, as has been stated, the amount was accurately determined 

 and allowed for, the greatest possible error in either direction arising 

 from occluded materials never exceeded a few thousandths of the 

 weight estimated, and might safely have been neglected in an ordinary 

 analysis. These errors, as we have stated, tend to balance each other ; 

 and their maximum effect is shown in the first two groups of determi- 

 nations in the table on pages 36-7. In the group of five determinations 

 8-12, we endeavored, as has been stated, so to regulate the conditions 

 that the opposite errors should balance each other ; and the very remark- 



* In one case, when no tartaric acid was used whatever, and when the anti- 

 mony was kept in solution by hydrochloric acid only, as mentioned on page 22, 

 we obtained a distinct carbonaceous residue, evidently from some organic 

 material which the precipitate like a mordant had absorbed. 



