444 PROCEEDINGS OF THE AMERICAN ACADEMY. 



several compounds, which might throw light upon the possible variability 

 of atomic weights. All these inducements caused us to accept with 

 alacrity the exceedingly kind offer from Professor Horace L. Wells of a 

 large quantity of very pure caesium material, with a view to determin- 

 ing its atomic weight with precision. 



II. Review of Earlier Determinations. 



Since the discovery of caesium by Bunsen and Kirchhoffin 1860 only 

 four investigations concerning its atomic weight have been made; two by 

 Bunsen * himself, one by Johnson and Allen, f and one by Godeffroy. t 



In 1861 Bunsen published the results of his first determination. The 

 material used in this investigation was recovered from 150 tons of water 

 taken from the mineral spring at Durkheim. The amount of caesium 

 obtained from this large quantity of water was only about twelve grams, 

 and the difficulty in purifying so small a portion probably accounts for 

 the inaccuracy of the results. Bunsen purified his material by recrystal- 

 lizing the chlorplatinate, which is not nearly as soluble as the corre- 

 sponding rubidium salt. The first value found by Bunsen for the atomic 

 weight of caesium was 132.35, but this number was soon afterwards 

 rejected even by himself, since his salt undoubtedly contained traces of 

 rubidium. 



A year later, Johnson and Allen, of the Sheffield Scientific School, 

 New Haven, having discovered a much richer source of caesium, began 

 their work on the atomic weight of this element. From the mineral 

 lepidolite found in Hebron, Maine, which contained about three per cent 

 of caesic oxide, they prepared a mixture of lithic, sodic, potassic, rubidic, 

 and caesic chlorides. On treating this mixture with chlorplatinic acid 

 the rubidium and caesium were thrown down as the corresponding chlor- 

 platinates. To separate the rubidium from the caesium, the chlorides 

 were converted through the carbonates into the bitartrates. The acid 

 tartrate of rubidium being only one-eighth as soluble as the correspond- 

 ing caesic salt, successive fractional recrystallization left the mother 

 liquor nearly free from rubidium. A portion of the salt thus obtained 

 was treated with chlorplatinic acid in quantity sufficient for complete 

 precipitation, and the caesic chlorplatinate thrown down was washed, 

 and reduced in a stream of hydrogen. 



* Bunsen, Zeit. anal. Chem., 1, 137 (1862) ; Pogg. Annal. 119, 1 (1862). 

 t Johnson and Allen, Am. Jour. Sci. and Arts (2), 35, 94. 

 } Godeffroy, Annal. der Chem., 181, 176 (1876). 



