1872.] Mr. W. Crookes on the Atomic Weight of Thallium. 481 



The reduction of the atomic weight from these data becomes a case of 

 simple proportion ; but the values found are absolute in so far only as the 

 atomic weights of nitrogen and oxygen are correct. The atomic weights of 

 nitrogen and oxygen have been usually represented by the numbers 14 and 

 16 ; but Professor Stas found these elements represented, according to ob- 

 servation, by 



Oxygen (0 3 ).. = 47*880 



Nitrogen =14*009 



or nitric acid N0 3 =61*889. According to the old equivalents, N0 6 =62. 



Taking as data the series of weighings in vacuo, the quantity of nitric 

 acid required to convert the thallium into nitrate is 



(239-646066-183790232 = ) 55*855834 grs. 



We have, then, with Professor Stas's determination of the atomic weights 

 of nitrogen and oxygen, the following proportion : — 



Weight of Weight of Atomic weight of Atomic weight 



nitric acid. thallium. nitric acid. of thallium. 



55-855834 : 183790232 :: 61-889 : x; 



.\ #=203-642. 



Let us see what would be the atomic weight of thallium if one or other 

 of the corrections introduced into the above determinations had been 

 omitted. The use of the old equivalent (=62) for nitric acid, with the 

 data derived from the weighings in vacuo, gives 



55-855834 : 183*790232 :: 62 : 204*007 

 as the atomic weight ; but I cannot admit this number to be so nearly 

 correct as 203*642. 



If we take the corrected weighings in air of ordinary density, we have, 

 with N9 3 =61*889, 



203- 738. 



WithN0 6 =62, 



204- 103. 



Accepting the uncorrected weights, observed in air, we have, with 

 NG 3 =61-889, 



203- 162. 



WithN0 6 =62, 



204- 165. 



The error of the last deduction, +-523, is sufficiently large to show the 

 necessity of neglecting no precaution in chemical manipulation, especially 

 in a determination of this character. The largeness of these errors have an 

 immediate bearing upon quantitative analysis ; for they show that from 

 data ordinarily given, very varying results may be obtained. Chemists 

 have to deal with much smaller quantities than a quarter per cent., par- 

 ticularly in organic analysis, where such a difference from the truth may 

 lead to very erroneous reasoning. 



vol. xx. 2 N 



