THE DISCOVERY OF THE ELEMENTS. 479 



of rubidium, and the basylity of rubidium inferior to that of caesium, 

 which is indeed the most powerfully basylous, or oxidizable, or elec- 

 tro-positive element known. 



Since 1860, both rubidium and caesium have been recognized as 

 minute constituents of a considerable number of minerals and mineral 

 waters, rubidium having been met with for the most part in a larger 

 proportion by weight than caesium. Unlike potash, originally known 

 as vegetable alkali, caesium has not been recognized in the vegetable 

 kingdom ; but rubidium has been found as a very common, minute 

 constituent of vegetable ashes, as those of beet-root, oak-wood, tobacco, 

 grapes, coffee, tea, etc. On the other hand, caesium, free from rubid- 

 ium, has been found in a tolerably well-known, though rare, mineral 

 from the island of Elba, to the extent of 32 per cent, by weight of the 

 mineral. The history of this mineral is curious : from the circumstance 

 of its always occurring in association with another mineral, a variety 

 of petalite, the two were called Castor and Pollux. Castor was found 

 to be substantially a silicate of alumina and lithia ; pollux a silicate of 

 alumina, and, as it was thought, of potash. The constituents of pollux, 

 namely, silica, alumina, and potash, with small proportions of ferric 

 oxide, lime, soda, and water, were duly estimated ; but the quantities 

 of these constituents, found in 100 parts of the mineral, instead of 

 amounting to 100 parts or thereabouts, amounted only to 88 parts, 

 there being somehow a loss of 12 per cent, in the analysis. After 

 Bunsen's discovery of the new alkali-metals, pollux was analyzed 

 afresh by Pisani, who soon perceived that what had formerly been 

 taken for potash, and estimated as potash, was not potash at all, but 

 caesia. Then calculating out his own analysis with caesia instead of 

 potash, substituting the one for the other in the proportion of 133 + 8, 

 or 141 parts of caesia, for 39 + 8, or 47 parts of potash, he found that 

 the quantities of the different constituents furnished by 100 parts of 

 the mineral yielded by their addition the full sum of 100 parts re- 

 quired. 



In submitting to spectroscopic examination a certain residue left 

 by the distillation of some impure selenium, Mr. Crookes, early in 1861, 

 recognized in the spectrum before him a brilliant-green line, from 

 which he inferred the presence in the above residue of a new element ; 

 and by the end of the same year he had succeeded in establishing the 

 tolerably wide distribution of this element, to which he gave the name 

 of thallium ; in procuring it, though but in small quantity, in a sepa- 

 rate state ; and in satisfying himself of its metallic character. Soon 

 afterward, and without knowledge of Mr. Crookes's later results, the 

 metal was obtained by M. Lamy, on a comparatively large scale, and 

 was exhibited by him in the form of small ingots at the London Exhi- 

 bition of 1862. He procured it from the fine dust met with in some oil- 

 of-vitriol factories, as a deposit in the flues leading from the pyrites 



