1872.] Mr. W. Crookes on the Atomic Weight of Thallium. 479 



flame ; this loss may amount to several thousandths of a grain in the space 

 of two hours when a bulb of Bohemian glass 3 inches in diameter is exposed 

 to a decided red heat in a gas-flame. Following the suggestion of Pro- 

 fessor Stas, I have obviated this source of error by employing a bath of 

 pure magnesia ; and I find that the weight remains constant even at a nearly 

 white heat. I have likewise employed baths of lime with similar satisfac- 

 tory results. 



The special apparatus that I have used are described in the processes in 

 which they were required ; I need scarcely say that in no case were materials 

 of untried purity employed. 



Improved Sprengel Vacuum-pump. 

 Before detailing the processes of the determination, it will be requisite to 

 describe the means of producing a vacuum in the flasks and bulbs em- 

 ployed. In proceeding with the determinations, several additions and 

 improvements have been made to the Sprengel pump as generally found in 

 the laboratory. The apparatus, as thus arranged, is readily manageable, 

 with certainty of obtaining a Torricellian vacuum. 



SECTION III. 



The Chemicals. 



The detail of the processes of preparing the thallium and the reagents in 

 a chemically pure state is too necessary to admit of useful abstraction. 



SECTION IV. 



Processes and Results. 



The processes and manipulation necessary to the determination of an 

 atomic weight are at all times difficult and delicate, but especially in the case 

 of a metal such as thallium, so readily oxidizable. This strong tendency to 

 combine with oxygen, renders the ordinarily exact processes of weighing out 

 pure metals inapplicable to the present purpose. The chances of contact 

 with the oxygen of the atmosphere must be reduced to a minimum, and to 

 this end it was found desirable to work in vacuo. For this purpose the series 

 of bulbs shown in fig. 1 1 have been blown. Pure thallium is introduced 

 into a, the upper end of which is then sealed. The end c is also sealed up, 

 and the horizontal tube e is connected to the Sprengel pump, and a 

 vacuum obtained, the tube being then sealed at f. The thallium in a is 

 now heated on a magnesia-bath to its point of fusion (561° F.), and when 

 molten caused, by gently tilting the vessel, to flow by the narrow channel 

 d into the lower bulb b, all oxide remaining in a. The channel d is 

 now contracted before the blowpipe, and a removed. The bulbs then 

 appear as in fig. 12, and after cooling are carefully weighed in air and 

 in vacuo. The fine point of a blowpipe-flame, caused to impinge upon the 

 end of the tube at g, softens the glass, and the air, endeavouring to force 



