1872.] Mr. W. Crookes on the Atomic Weight of Thallium. 483 



The probable error is 0*0022 ; and the probability that the true value lies 

 between 203*632 and 203*652 is 99808, certainty being represented by 

 unity. 



I may therefore conclude that the atomic weight of thallium is, within 

 the limits of error (as small as possible) of observation, — 



203-642. 



Professor Stas has shown the hypothesis of Prout — that the atomic 

 weights of the elements are severally multiples of the atomic weight of 

 hydrogen — to be without the corroboration of experimental result. This 

 view of the hypothesis is further borne out in the present investigation ; for 

 the number 203*642 cannot, within the limit of what has been shown to be 

 the probable error, by any liberty be made to follow the hypothesis. 

 Without doubt, when the atomic weights of all the metals are redetermined 

 according to the standard of recent scientific method, it will be found that 

 there are more exceptions to the hypothesis than commonly considered. 

 Marignac gives, in his confirmatory discussion of Stas's experiments, and in 

 his own results with calcium (40*21), lanthanum (94*13), strontium (87*25), 

 analogous opposed evidence, as in the case of the weight found for 

 thallium. 



I have thus striven to eliminate all erroneous influence in the number I 

 submit to the Royal Society as the atomic weight of thallium ; and I shall 

 be amply rewarded for my long labour if I can know that the determina- 

 tion has secured to researches of this character a nearer approach to the 

 standard of truth. 



The drawings appended to the complete paper comprise copies of : — 



Fig. 1. The case of the vacuum-balance. 



2. The improved Sprengel vacuum-pump. 



3. Apparatus for preparing pure water under diminished pressure. 



4. Apparatus for preparing nitric acid under diminished pressure. 



5. Apparatus for preparing oxalic acid under diminished pressure. 



6. Apparatus for preparing sulphuric acid under diminished pressure. 



7. Stoppered tube in which thallium was weighed in the early de- 



terminations. 



8. A series of bulbs*. 



9. Another series of bulbs*. 



10. Apparatus for sealing up thallium in hydrogen. 



11. The first stage of selected glass apparatus*. 



12. The second stage of selected glass apparatus*. 



13. The Buusen water-pump and drying-apparatus. 



14. The third stage of selected glass apparatus*. 



For the conversion of thallium into nitrate of thallium. 



2 n 2 



