2 LankSHEAR, Quantitative Absorption Spectra. 



others has led to some useful generalisations, yet here 

 again no really satisfactory theory of cause has yet been 

 put forward. One reason, and perhaps the chief one, for 

 the present state of affairs, is that the method of examin- 

 ing colours has been largely qualitative. 



In the following pages a brief description of the 

 method at present in use for the study of ultraviolet 

 absorption spectra is given, together with a short summary 

 of the more important generalisations arrived at. A new 

 instrument is then described which permits quantitative 

 measurements of the amount of light absorbed by a given 

 quantity of a substance. 



The present, or Hartley method, was suggested by 

 Hartley in 1 879 {Proc. Roy. Soc., 28, p. 233). It has been 

 considerably developed by Dobbie, Baly, Purvis and 

 others. 



The Hartley Method. 



Apparatus. The apparatus used consists of a source 

 of ultraviolet light, a quartz condenser, a suitable quartz- 

 ended containing vessel for the substance under examina- 

 tion, and a quartz spectrograph in which the transmitted 

 light is allowed to fall on a photographic plate after 

 dispersion. 



Source of Light. A spark from a six inch induction 

 coil is passed between suitable electrodes with a con- 

 denser in parallel. An iron arc is also sometimes used. 

 The choice of the electrodes is a matter of some difficulty, 

 but the Jones' form is the most serviceable. This con- 

 sists of carbon poles impregnated at a red heat with 

 uranium acetate and ammonium molybdate. The tips 

 are wedge-shaped, and the long edges are placed parallel 

 to the optic axis. The spark gap is about 5 mm. The 

 spectrum of this spark is very rich in ultraviolet light, and 



