280 
ME. W. CEOOKES ON THE ATOMIC WEIGHT OF THALLIUM. 
avoid the imputation of over-refinement in this research ; but considering the fallibility 
of human operations, and especially those of so complicated a nature as I am about to 
describe, I have considered it better to err on the side of too great than of too little 
precaution, both in the purification of the chemicals, the arrangement of the apparatus, 
the time devoted to each separate determination, the removal of the errors incidental 
to the weighings, and the subsequent calculations. These latter have been especially 
tedious, as the numbers have generally extended to too many places of figures to allow 
the use of logarithms ; each calculation has, moreover, been duplicated by different 
persons. 
I have attempted two entirely different methods of arriving at the atomic Aveight of 
thallium. Had the results of these determinations differed materially, I should have 
extended the research to other methods ; but as they nearly agree it appeared unne- 
cessary to incur so great an additional expenditure of time and material with no reason- 
able prospect of getting any but confirmatory results. 
The first method, and that Avhich I shall describe, consists in taking a known quantity 
of metallic thallium, dissolving it in nitric acid, and weighing the nitrate of thallium 
produced. 
The second method consists in dissolving known quantities of sulphate of thallium in 
water, and ascertaining how much nitrate of barium is necessary to precipitate the sul- 
phuric acid as sulphate of barium. 
In the prosecution of these tAvo methods, the materials employed, the transferences 
from one vessel to another, and the weighings are reduced to a minimum, Avliile several 
precautions have been introduced into the operations of weighing which are not usually 
adopted. No correction has been neglected that is not many times less than the pro- 
bable error of a single obseiwation ; and, as I have stated, especially has attention been 
paid to such corrections as ahvays influence in one direction, as in that for Aveight of air 
displaced. Errors sometimes in excess and sometimes in defect tend to disappear from 
the mean of a great number of observations. 
I have for the foregoing reasons thought it necessary to dAvell thus far upon the care 
I have bestoAved upon my Avork. In the succeeding section I shall describe accurately 
the apparatus employed, including the balance and Aveights, and the necessary arrange- 
ments for weighing in vacuo. In the third section I shall enumerate the chemicals and 
the methods of preparing them and pure thallium. The fourth section will be devoted 
to the process determining the atomic Aveight and the Aveights obtained. The concluding 
section Avill consist of a calculation and discussion of results. 
Section II.— APPAEATUS EMPLOYED. 
The absolute weight of any substance may be found by calculation from its apparent 
weight in an atmosphere balancing 30 inches of mercury, and from its apparent weight 
under, say, 25 inches of mercury ; but the errors of observation, more especially those 
relating to the maintaining of a partial vacuum, Avill largely affect the result. Weighings 
