196 JONES-STRONG— THE ABSORPTION SPECTRA [April 24, 



sulphate and sulphocyanate of cobalt, the chloride, acetate and sul- 

 plate of nickel, the chloride, sulphate and acetate of chromium, 

 chrome alum, the nitrate and bromide of copper, uranous chloride, 

 erbium chloride, the chloride and nitrate of prsesodymium, the sul- 

 phate, acetate, chloride and nitrate of uranyl and the chloride, 

 bromide and nitrate of neodymium. Spectrograms are made of the 

 absorption spectra for a given concentration of a salt, keeping the 

 thickness of layer constant for every 15° between 0° and 90° C. 



To make a spectrogram light from a 'Nernst glower and from a 

 spark is allowed to pass through the solution that is being in- 

 vestigated. It is then focused upon the slit of a spectroscope — and 

 falling then on a concave grating, the light is spread out into a 

 spectrum on the film upon wnich it is photographed. The films used 

 were made by Wratten and Wainwright of Croyden, England, and 

 were very uniformly sensitive to light from A 2100 to A 7200. 



The sectional diagram (Fig. i) will make the experimental 

 arrangement of the apparatus clearer. N is a Nernst glower which 

 is arranged to slide along the rod AB. P and P' are quartz prisms 

 which are held by a lid L. The prism P is stationary, whereas the 

 prism P' can be moved by the travelling carriage E back and forth 

 through the trough T which contains the solution whose absorption 

 spectrum is being investigated. AB is so inclined that the optical 

 length of the light beam from A'' to P', P and the concave mirror M 

 shall be constant, whatever the length of the solution between P 

 and P' may be. The greatest length of path PP' used was 200 mm. 

 The hypothenuse faces of P and P' are backed by air films which 

 are enclosed by glass plates cemented to the quartz prisms. 



Considerable difficulty was experienced in finding a cement that 

 would adhere to the polished quartz prisms at the higher tempera- 

 tures. For aqueous solutions baked caoutchouc was found to work 

 fairly well. D is a brass box holding the trough T. D is filled with 

 oil and is placed in a water-bath whose temperature can be varied 

 between 0° and 90° C. The path of a beam of light is then from 

 the Nernst glower (N) or spark to the quartz prism P'. The light 

 is totally reflected from the hypothenuse face of this prism through 

 the solution to P. This prism also has its hypothenuse face backed 

 by an air-film, so that the light is totally reflected upwards to the 



