POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIUM 573 



no reason for supposing that the spectrum of a compound is equal to 

 the sum of the spectra of its elements that is, every compound which 

 is not decomposed by heat has its own proper spectrum. This is best 

 proved by absorption spectra, which are essentially only reversed spectra 

 observed at low temperatures. If every salt of sodium, lithium, and 

 potassium gives one and the same spectrum, this must be ascribed 

 to the presence in the flame of the free metals liberated by the 

 decomposition of their salts. Therefore the phenomena of the spectrum 

 are determined by molecules, and not ly atoms that is, the molecules 

 of the metal sodium, and not its atoms, produce those particular 

 vibrations which determine the spectrum of a sodium salt. Where 

 there is no free metallic sodium there is no sodium spectrum. 



Spectrum analysis has not only endowed science with a knowledge 

 of the composition of- distant heavenly bodies (of the sun, stars, 

 nebulae, comets, &c.), but has also given a new method for study- 

 ing the matter of the earth's surface. With its help Bunsen discovered 

 two new elements belonging to the group of the alkali metals, and 

 thallium, indium, and gallium were afterwards discovered by the same 

 means. The spectroscope is employed in the study of rare metals 

 (which in solution often give distinct absorption spectra), of dyes, 

 and of many organic substances, &c. 37 With respect to the metals 

 which are analogous to sodium, they all give similar very volatile 



their light may be examined by placing the apparatus in front of the slit of a spectro- 

 scope. The variations to which a spectrum is liable may easily be observed by increasing 

 the distance between the wires, altering the direction of the current or strength of the 

 solution, &c. 



3 ? The importance of the spectroscope for the purpose of chemical research was 

 already shown by Gladstone in 1856, but it did not become an accessory to the laboratory 

 until after the discoveries of Kirchhoff and Bunsen. It may be hoped that in time 

 spectroscopic researches will meet certain wants of the theoretical (philosophical) 

 side of chemistry, but as yet all that has been done hi this respect can only be regarded 

 as attempts which have not yet led to any trustworthy conclusions. Thus many investi- 

 gators, by collating the wave-lengths of all the light vibrations excited by a given element, 

 endeavour to find the law governing their mutual relations ; others (especially Hartley 

 and Ciamician), by comparing the spectra of analogous elements (for instance, chlorine, 

 "bromine, and iodine), have succeeded in noticing definite features of resemblance in, 

 them, whilst others (Griinwald) search for relations between the spectra of compounds 

 and their component elements, &c. ; but owing to the multiplicity of the spectral lines 

 proper to many elements, and (especially in the ultra-red and ultra-violet ends of the 

 .spectrum) the existence of lines which are undistinguishable owing to their faintness, 

 and also owing to th comparative novelty of spectroscopic research this subject cannot 

 "be considered as in any way perfected. Nevertheless, in certain instances there is 

 evidently some relationship between the wave-lengths of all the spectral lines formed by 

 a given element. Thus, in the hydrogen spectrum the wave-length =364'542 w 2 /(ra 8 4) 

 if m varies as a series of whole numbers from 8 to 15 (Walmer, Hagebach, and others). 

 For example, when m = S, the wave-length of one of the brightest lines of the hydrogen 

 spectrum is obtained (656'2), when m = 7,one of the visible violet lines (3968), and when 

 m is greater than 9, the ultra-violet lines of the hydrogen spectrum. 



