POTASSIUM, KUBIDJl'.M. CAKSlt'M. AND LlTHir.M 



565 



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 

 Imx its oi<-)t pi'npi / sjircfrtu/i. 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 phe- 

 nomena of tlf K/'f-t ni mare determined by molecules, and not by atoms 

 that is, the molecules of the metal sodium, and not its atoms, produce 

 those forms of vibrations which are expressed in the spectrum of a 

 sodium salt. Where there is no free metallic sodium there is no 

 sodium spectrum. 



Spectr a in ium/i/fiis has not only endowed science with a represent- 

 ation of the composition of the 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. Bunsen by its means 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 

 in general of many organic substances, &c. 37 With respect to the 



A a (from 0'3 to 0\") mm. in diameter) is fused, is immersed in a narrow cylinder, C (in, 

 which it is firmly held by a cork). The projecting end, a, of the 

 wire is covered by a fine capillary tube, d, which extends 1-2 mm. 

 beyond the wire. Another straight capillary tube, E, with a plati- 

 num wire, B Z>, about 1 mm. in diameter (a finer wire soon becomes 

 hot), is held (by a cork or in a stand) above the end of the tube, 

 D. If the wire A be now connected with the positive, and the 

 wire B with the negative, terminal of a Ruhmkorff's coil (if the 

 wires be connected in the opposite order, the spectrum of air is 

 obtained), a series of sparks rapidy following each other appeal- 

 between a and b, and their light may be examined by placing the 

 apparatus in front of the slit of a spectroscope. The variations to 

 which a spectrum is liable may easily be observed by increasing 

 the distance between the wires, altering the direction of the cur- 

 rent or strength of the solution, &c. 



37 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 explain certain wants of the 

 theoretical (philosophical) side of chemistry, but as yet all that 

 has been done in this respect can only be regarded as attempts 

 which have not yet led to any trustworthy conclusions. Thus many, 

 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 



OB 



FIG. 76. Method of 

 showing the spec- 

 trum of substances 

 ill solution. 



