476 Mr. W. Crookes on the Atomic Weight of Thallium. [June 20, 



characteristics and chemical properties ; also I discussed the position of 

 thallium among elementary bodies, and gave a series of analytical notes. 



In the pages of the ' Journal of the Chemical Society ' for April 1, 1864, 

 I collated all the information then extant, hoth from my own researches 

 and from those of others, introducing qualitative descriptions of an extended 

 series of the salts of the metal. I propose in the present paper to lay 

 before the Royal Society the details and results of experiments which have 

 engrossed much of my spare time during the last eight years, and which 

 consist of very laborious researches on the atomic weight of thallium. 



SECTION I. 

 On the Determination of Atomic Weights. 



In determining accurately the atomic weight of a metal that stands so 

 high in the scale as thallium, difficulties and sources of error which are 

 comparatively small with elements of low atomic weight are magnified to 

 serious proportions, and require more than ordinary care for their elimina- 

 tion. When so large a proportion of the compound under analysis or 

 synthesis consists of the body itself whose atomic weight is the one un- 

 known quantity, it is evident that the almost unavoidable errors occasioned 

 by impurity in the materials employed, the losses incident to imperfect 

 manipulation, or the inaccuracies arising during the weighing from the 

 omission of the corrections required by temperature, pressure, &c, will all 

 find their way into the number which is finally considered to represent the 

 atomic weight of the metal. 



I have attempted two entirely different methods of arriving at the 

 atomic weight of thallium. Had the results of these determinations 

 differed materially, I should have extended the research to other methods ; 

 but as they so nearly agree, it appeared unnecessary to incur so great an 

 additional expenditure of time and material with no reasonable prospect 

 of getting any but confirmatory results. The first method, and that which 

 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 sulphuric acid as sulphate of barium. 



SECTION II. 



Apparatus employed. 



The absolute weight of any substance may be found from its apparent 

 weight in an atmosphere of 30 inches of mercury, and from its apparent 

 weight under, say, 25 inches of mercury. But the best weighings are 

 undoubtedly one in air at ordinary pressure and temperature, and one in a 



