THALLIUM. 185 



One hundred parts of Agl correspond to the amounts of Til given in 

 the last column : 



.720 grm. Til gave .51 grm. Agl. 141.176 



2.072 " 1.472 " 140.761 



.960 " .679 " 141-384 



3 8 5 -273 " 141.026 



1.068 " .759 " 140.711 



Mean, 141.012, .085 



In the second series the thallium iodide was decomposed by ammonia 

 in presence of silver nitrate, and the resulting Agl was weighed. Ex- 

 pressed according to the foregoing standard, the results are as follows : 



1.375 grm. Til gave .978 grm. Agl. Ratio, 140.593 



1.540 1.095 " " 140.639 



1.380 " .981 " " 140.673 



Mean, 140.635, db .016 



General mean of both series, 140.648, .016. 



In 1873 Crookes,* the discoverer of thallium, published his final deter- 

 mination of its atomic weight. His method was to effect the synthesis of 

 thallium nitrate from weighed quantities of absolutely pure thallium. 

 No precaution necessary to ensure purity of materials was neglected ; the- 

 balances were constructed especially for the research ; the weights were 

 accurately tested and all their errors ascertained ; weighings were made 

 partly in air and partly in vacuo, but all were reduced to absolute stand- 

 ards ; and unusually large quantities of thallium were employed in each 

 experiment. In short, no effort was spared to attain as nearly as possi- 

 ble absolute precision of results. The details of the investigation are too 

 voluminous, however, to be cited here ; the reader who wishes to become 

 familiar with them must consult the original memoir. Suffice it to say 

 that the research is a model which other chemists will do well to copy. 



The results of ten experiments by Professor Crookes may be stated as 

 follows. In a final column I give the quantity of nitrate producible 

 from 100 parts of thallium. The weights given are in grains : 



* Phil. Trans., 1873, p. 277. 



