266 



of waves expressed in millionths of a millimetre in order to get abso- 

 lute numbers immediately comparable with those of others. 



The discharge through a capillary tube containing residual traces 

 of pure hydrogen is of a deep red colour ; the spectrum obtained from 

 it consists of only three bright bands : of a most splendid red one, 

 Ha; of a bluish-green one, H/3, nearly as bright ; and of a fainter violet 

 one, Hy. The following table contains the angle of refraction 0, the 

 index of refraction yu, and the length of the wave X, corresponding to 

 each of the three rays Ha, H/3, Hy, as well as of the dark lines of 

 Fraunhofer, C, F, G, in order to compare their reciprocal position : 



Ha.... 57 H)'-5 C.... 57 8'-5 



H/3 59 55-5 F.... 59 55-5 



Hy.... 61 43 G.. . 61 55-5 



Ha.... 653-3 C 656-4 



H/3.... 484-3 F...... 484-3 



Hy 433-9 G 429-1 



From this table it results that H/3 exactly coincides with F, while 

 Ha and Hy approach very near C and G. 



The spectrum of pure oxygen was only obtained after several un- 

 successful trials to procure proper tubes, the gas being in most cases 

 absorbed by the electrode during the passage of the current. 



The spectrum of nitrogen is one of the richest in colours. Its 

 less refrangible part is of a peculiar nature, having, from the exterior 

 red to the limit of the yellow, seventeen equidistant small dark bands. 



Most characteristic are the spectra of carbonic acid (oxide of 

 carbon), iodine, bromium, chlorine, chloride of silicium, chloride of 

 phosphorus, chloride of tin. The spectrum of the last-mentioned 

 substance is one of the most remarkable. The colour of the gas 

 within the larger parts of the tube through which RuhmkorfFs 



