SECT. ii. THE LONG SPECTRUM. 63 



light, while calorescence causes the molecules of a sub-i 

 stance to oscillate in longer, periods than the incident \ -> 

 light. The refrangibility of the rays is lowered in the/ 

 first case, and raised in the second. 



Substances differ as much in their transmission of 

 the chemical rays as those of light and heat. Glass 

 is impervious to the most highly refrangible chemical 

 rays, while rock crystal transmits them with the . 

 greatest facility; and on that account the absolute 

 length of the spectrum was not known till the light 

 was refracted by prisms of rock crystal. Besides, the 

 number, position, and intensity of the chemical rays 

 vary with the source of light. Some flames have scarcely 

 any chemical rays ; that of the oxy-hydrogen blowpipe, 

 though intensely hot, has very few, and even the solar 

 light is inferior in that respect to electricity. The 

 electric spark from the prime conductor of a common 

 electrifying machine, or the discharge of a Ley den jar, 

 emits rays of very high refrangibility, far surpassing 

 those which emanate from the sun. For, when the 

 electric light from a highly charged Ley den jar was 

 refracted by two quartz prisms and thrown by Pro- 

 fessor Stokes on a plate of uranium glass, the chemical^ 

 spectrum was highly luminous, and six or eight times \ 

 as long as the visible spectrum. An equally extensive 

 spectrum was obtained from the voltaic arc taken 

 between copper points; it consisted entirely of bright 

 lines. The long spectrum also appeared on the uranium 

 glass when the spark refracted by quartz prisms was 

 obtained from the secondary terminals of an induction 

 coil in connection with the coatings of a Ley den jar. 

 It consisted of bright lines, but was not so luminous as 

 that from a powerful voltaic battery. On changing the 

 metals of the points between which the sparks passed, 

 the bright lines were changed, which showed that they 

 were due to the particular metals. 



