HISTORY OF SCIENCE. 



at Diirkheim, he observed certain lines which he had never seen in 

 spectra before. He was so convinced that these lines were due to 

 minute quantities of some hitherto unknown chemical elements, that, 

 in order to separate from the water a quantity of the substances suffi- 

 cient to admit of their examination by ordinary methods, he had no 

 less than 44 tons of the Diirkheim water evaporated. He was able 

 to isolate from the residue salts of two hitherto unknown metals, of the 

 same class as potassium, sodium, and lithium. To one of these metals 

 he gave the name of rubidium, because among the lines of its spectrum 

 were two very intense red lines. The other metal, on account of two 

 splendid blue lines in the spectrum of its compounds, was called ca- 

 sium. The quantities of the salts of these two metals contained in 

 the Diirkheim water are extremely small. One ton of water contained 

 only 4 grains of chloride of rubidium and 3 grains of chloride of caesium, 

 These quantities are mere traces, which could never have been recog- 

 nized by any ordinary method of examination of mineral waters. So 

 great is the delicacy of the spectroscopic reactions, that one-millionth 

 part of a grain of chloride of caesium can be recognized by its charac- 

 teristic lines. Singularly enough, rubidium, though existing in very 

 small quantities, was found to be widely diffused in the vegetable 

 world. It has been found in the ashes of tea, coffee, cocoa, beetroot, 

 oak, and other plants. 



Another metal which yields a very characteristic spectrum, and which 

 was, before these researches, supposed to be of very rare occurrence, was 

 found to be in reality one of the most widely diffused of the chemical 

 elements. So far is lithium the metal to which we allude from being 

 confined to half a dozen minerals, in which it was previously known ; 

 that it is found in almost every rock, in the ashes of most plants, in 

 sea and river-water, and in animals. The spectrum of lithium is cha- 

 racterized by a splendid red line of great intensity. There is also a 

 feeble yellow line, and these two are the only lines which the spectrum 

 of this substance presents. As in the case of the other alkaline com- 

 pounds, the spectrum reactions of lithium are of great delicacy, for so 

 small a quantity as ^ o-owo tn P art f a g ram can easily be detected. 

 Indeed, the spectroscopic reactions of the compounds of all the al- 

 kalies and alkaline earths are remarkable for the extremely small 

 quantities of the substance required to exhibit the characteristic lines. 

 Whether their spectra exhibit few or many lines, these fall in each 

 case in different parts of the spectrum ; and when once the positions 

 of the lines have been mapped, the spectrum and the substance pro- 

 ducing it can be identified with absolute certainty. 



The year following the discovery of the two new metals a third was 

 found by Mr. W. Crookes by the spectrum indications in a mineral 

 obtained from the Hartz. This substance imparted a bright green tint 

 to flames, and Mr. Crookes found that the flames gave a spectrum 

 consisting of a single line of a beautiful green colour. He isolated 



