224 



THE POPULAR EDUCATOR. 



Fig. 60. 



are single lines in tha red portion of the spectrum ; D is a 

 double line at the confines of the orange and yellow ; E is a 

 group of fine lines in the green ; F is a strongly marked line in 

 the blue ; Q is a group of finely marked lines in the indigo, and 

 H is a similar line in the violet. These seven lines are of the 

 utmost importance in optical researches, being 

 the " constants ' ' of the spectrum. 



For many years the meaning of these lines 

 was undiscovered, until, chiefly by the re- 

 searches of Bunsen and Kirchhoff , they have 

 proved to be the elements of what is now 

 called spectrum analysis. 



Frauiihofer's lines were evidently inde- 

 pendent of the prisms used, for, however the 

 spectrum was procured, there the lines ap- 

 peared exactly in the same positions. Hence 

 it was rightly concluded that they were 

 belonging to the light itself, and were spaces 

 in which, for some cause or 

 other, the visual rays were 

 totally absorbed. When it was 

 attempted to treat light from 

 other sources in the same way 

 as the sunlight, a general prin- 

 ciple was soon laid down, that 

 no body would give any but a 

 continuous spectrum unless it 

 was heated to such a tempera- 

 ture that it was vaporised, and 

 a spectrum which would exhibit 

 the lines must be from lumi- 

 nous vapour. But the spectra Fig. 59. 

 of bodies thus studied presented 



a very marked difference to the solar spectrum, in that the 

 lines which each substance produced were not black, but 

 luminous, and of the colour which corresponded to that part of 

 the solar spectrum in which they were situated, the rest of the 

 spectrum being very faintly exhibited. For example, the sodium 

 spectrum shows only one bright yellow line, 

 which occupies the position of Fraunhofer'a 

 lino D. Potassium gives three lines, two red 

 and one violet. 



If now we burn a mixture of potassium and 

 sodium, in the resulting spectrum these four 

 lines appear, immediately declaring the pre- 

 sence of the two metals. The spectra of all 

 the elements has been examined, and it is 

 confidently expected that the time will come 

 when by a glance at the spectrum of any 

 body in a proper state of 

 combustion, its composition 

 will be fully and accurately 

 determined. This method 

 of analysis has not as yet 

 reached perfection, and a 

 large field is offered for ex- 

 perimentalists. 



One of the simplest spec- 

 troscopes is shown in Fig. 

 61. The body under con- 

 sideration is held in the 

 flame of a Bnnsen's burner. 

 The light given off by its 

 combustion is admitted into 

 the tube, c, through a slit 

 made by the edges of two 

 sliding brass plates, which 

 are opened or shut by means 

 of a screw ; in the tube is a 



lens by which the rays of light are prevented from being scat- 

 tered, and brought into a parallel pencil. The rays now pass 

 through the prism E, whereby the spectrum is produced. This 

 is magnified, and therefore better examined by the telescope B. 



In many spectroscopes, by means of a side aperture, a pencil 



of sunlight is introduced, and by a simple arrangement the 



solar spectrum is placed in juxtaposition to that of the body 



under examination, and thus a close comparison is effected. 



It will be noticed that by this method there is at present no 



Fig. 5P. 



attempt at quantitative analysis, but it is only useful iu deter- 

 mining the elements present in any body; in this it is pre- 

 eminently successful, for its delicacy can appreciate the presence 

 of many millionths of a grain. By its means many elements 

 which were supposed to be of rare occurrence are found to be 

 widely distributed. Lithia, for example, had 

 only been detected in four minerals previous 

 to the application of this new analytical 

 method. Now it is known to exist in minute 

 quantities in tobacco, milk, blood, and most 

 spring waters. We owe to the spectrum 

 analysis the discovery of four new elements 

 rubidium, caesium, thallium, and iridium. 



There still remains an important point to be 

 noticed How is it that the lines in the solar 

 spectrum arc dark, while those in other spectra 

 are bright ? This difficulty, which for long 

 besot the subject, admits of a perfect explana- 

 tion. If a ray of white light be 

 passed through a flame in which 

 sodium is burning, instead of 

 the bright line J>, which is cha- 

 racteristic of sodium, a dark lino 

 is found to occupy its place in 

 the spectrum of the white light. 

 This is one of a class of experi- 

 ments from which the following 

 law has been deduced That 

 when a ray of light passes 

 through any substance in a state 

 of vapour, those rays are ab- 

 sorbed which that substance 

 would emit if it were luminous. 



So that the existence of these black lines in the solar spectrum 

 is a proof that all the bodies to whose existence the lines are 

 duo, are in a state of vapour in the atmosphere of the sun, and 

 the white light passing through the vapour is deprived of those 

 very rays which the bodies themselves are capable of emitting. 



This wonderful means of analysis enables 

 the chemist to leave our earth and examine 

 those stars of whose distance and size we have 

 little idea. The spectra of the stars, which are 

 suns of other systems, have also dark lines. 

 Many of our terrestrial elements have been 

 detected in their atmospheres, but there are 

 many lines which do not correspond to any of 

 the solar lines, which seem to indicate the 

 existence of bodies beyond our experience. 

 The spectra of some of the nebulae, have bright 

 lines, thus indicating that 

 these bright patches in the 

 heavens are masses of glow- 

 ing vapour, and are not either 

 solids or liquids surrounded 

 by luminous atmospheres. 

 The lines in the stellar 

 spectra promise yet to afford 

 other information, for by 

 their convergence it is be- 

 lieved that the motion of 

 the star may be determined. 

 The whole subject is fraught 

 with the deepest interest, 

 and promises to be one of 

 the richest fields of scientific 

 research. 



We have thus rapidly 

 passed over the vast field of 

 Nature, endeavouring to set 



before our readers some of the properties of the materials of 

 which her kingdom is composed. We commend the subject, not 

 only for its commercial value, but for its own intrinsic interest, 

 and as one of the most direct of those paths which lead the 

 serious student to the presence of that Being who is the source 

 of all truth ; ever reminding our readers that he who obtains 

 truth, and does not mingle with it the acquaintance of a living 

 God, possesses a soulless, dead thing, which can never enrich 

 him in that future state where we arc assured " God is all in all." 





