568 PRINCIPLES OF CHEMISTRY 



of the relation between the waves of light emitted and absorbed by a- 

 substance under given conditions of temperature ; this is expressed 

 by KirchhofFs law, discovered by a careful analysis of the phenomena. 

 This law may be formulated in an elementary way as follows : At a 

 given temperature the relation between the intensity of the light emitted 

 (of a definite wave-length) and the absorptive capacity with respect to the 

 same colour (of the same wave-length) is a constant quantity. 30 As a 

 black dull surface emits and also absorbs a considerable quantity of heat 

 rays whilst a polished metallic surface both absorbs and emits but few, 

 60 a flame coloured by sodium emits a considerable quantity of yellow 

 rays of a definite refrangibility, and has the property of absorbing a 

 considerable quantity of the rays of the same refractive index. In 

 general, the medium which emits definite rays also absorbs them. 



Thus the bright spectral rays characteristic of a given metal may 

 be reversed that is, converted into dark linesby passing light which 

 gives a continuous spectrum through a space containing the heated 

 vapours of the given metal. A similar phenomenon to that thus arti* 

 ficially produced is observed in sunlight, which shows dark lines 

 characteristic of known metals that is, the Fraunhofer lines form an 

 absorption spectrum or depend on a reversed spectrum ; it being pre- 

 supposed that the sun itself, like all known sources of artificial light, 

 gives a continuous spectrum without Fraunhofer lines. 31 We must 



yellow sodium colour), and the hydrogen is ignited in two burners in one large one 

 with a wide flame giving a bright yellow sodium light, and in another with a small fine 

 orifice whose flame is pale : this flame will throw a dark patch on the large bright flame. 

 In Ladoffsky's method the front tube (p. 561) is unscrewed from a spectroscope directed 

 towards the light of a lamp (a continuous spectrum), and the flame of a spirit lamp 

 coloured by a small quantity of NaCl is placed between the tube and the prism ; a black 

 band corresponding to sodium will then be seen on looking through the ocular tube. 

 This experiment is always successful if only there be the requisite relation between the 

 strength of light of the two lamps. 



50 The absorptive capacity is the relation between the intensity of the light (of a 

 given wave-length) falling upon and retained by a substance. Bunsen and Roscoe 

 showed by direct experiment that this ratio is a constant quantity for every substance. 

 If A stand for this ratio for a given substance at a given temperature for instance, for 

 a flame coloured by sodium and E be the intensity of the light of the same wave-length 

 emitted at the same temperature by the same substance, then Kirchhoff's law, the ex- 

 planation and deduction of which must be looked for in text-books of physics, states that 

 the fraction AlE is a constant quantity depending on the nature of a substance (as 4 

 depends on it) and determined by the temperature and wave-length. 



31 Heated metals begin to emit light (only visible in the dark) at about 420 (vary- 

 ing with the metal). On further heating, solids first emit red, then yellow, and lastly 

 white light. Compressed or heavy gases (see Chapter III., Note 44), when strongly heated, 

 also emit white light. Heated liquids (for example, molten, steel or platinum) also give 

 ft white compound light. This is readily understood. In & dense mass of matter the 

 collisions of the molecules and atoms are so frequent that waves of only a few definite 

 lengths cannot appear ; the reverse is possible in rarefied gases or vapours. 



