lxxii 



The fact that the rays from an incandescent metal are absorbed with 

 great energy by the vapor of the same metal, seems to be an additional 

 proof that the periods of vibration of the atoms of the metal, and those of 

 the vapor are not altered in rapidity by the change from the incandescent 

 to the vaporous form, but only in amplitude of vibration. 



Roscoe tells us that the spectroscope has shown that common salt or 

 chloride of sodium is the most widely disseminated substance known in 

 the world. The mist raised from the agitated surface of the ocean by the 

 winds is carried up into the aerial regions; the watery portions are soon 

 evaporated and the infinitely minute particles of salt are left suspended in 

 the air, and by it are borne over the most distant regions of the earth. 

 (This action of the winds must not be confounded with the evaporation 

 caused by the sun, for this does not carry up the salt.) These salt atoms 

 seem constantly present in dust, and Roscoe asserts that a book laying for 

 two hours on a table will collect sufficient to be detected by a spectroscope, 

 if the dust be brushed off and burned in the light of the instrument. The 

 one hundred and eighty millionth part of a grain thus burned will produce 

 visibly the bright yellow D line of the spectrum ! 



To detect the suspected presence of a metal or earth in any substance 

 under analysis, two separate beams are used to produce two different 

 spectra, one being by an arrangement of the spectroscope, shown imme- 

 diately above the other. If the presence of copper, for example, is sus- 

 pected in the substance to be examined, a piece of copper is burned, and 

 a piece of the substance to be analyzed also. The vapor from each is 

 brought into the paths of each of the beams of light at the same moment, 

 The characteristic green lines of copper will appear at once in the one 

 spectrum; and if copper be present, in the minutest quantity, in the sub 

 tance to be examined, the other spectrum will be marked with lines iden- 

 tically the same in color, position, and number; if it be absent, these lines 

 cannot possibly be produced. The spectra are viewed through the tele- 

 scope forming part of the instrument, and by it they arc magnified and are 

 thus thrown upon the retina. 



Bunsen in i860, having occasion to examine the alkaline earths contain- 

 ed in the waters of two springs at Durkheim, observed that their vapor gave 

 certain bright lines in the spectrum never before observed, and he at once 

 argued that they must have been caused by the presence of some unknown 

 metals. 



Acting upon this idea, this eminent chemist evaporated forty-four tons 

 of the water, and obtained from it two hundred grains of the new metals, 

 Caesium and Rubidium. Cresium produces two beautiful blue lines in 

 the spectrum, and Rubidium, as its name implies, two bright red ones. It 

 is found in many vegetables, especially in tobacco, and in beets, and also 

 in certain minerals. 



More recently, Mr. Crooks, in England, discovered by the spectroscope 

 another new metal, Thallium. This metal produces a bright green line 

 in the spectrum. Another new metal. Indium, has since been similarly 

 discovered. It produces a bright blue line. 



