CHAPTER II. 



EXPERIMENTAL METHODS. 



In this work the methods used by Jones and Uhler ' and Jones and 

 Anderson 2 have in the main been employed. 



The investigations of the effect of changes in temperature on the 

 absorption spectra of solutions have been confined to different concentra- 

 tions of aqueous solutions of the chloride, nitrate, acetate, sulphate, and 

 sulphocyanate of cobalt, the chloride, acetate, and sulphate 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 praseodymium, the sulphate, acetate, chloride, and nitrate of 

 uranyl, and the chloride, bromide, and nitrate of neodymium. Spectro- 

 grams of the absorption spectra for a given concentration of a salt, with a 

 constant thickness of layer, have been made for every 15 between 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 investigated. It is 

 then focused upon the slit of a spectroscope, and, falling on a concave 

 grating, is spread out into a spectrum on the film upon which it is photo- 

 graphed. The films used were made by Wratten and Wainwright, of 

 Croyden, England, and were very uniformly sensitive to light from X 2100 

 to ^ 7200. 



The sectional diagram (fig. 1) will make the experimental arrange- 

 ment 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 

 traveling 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 N 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 

 inclosed 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 temperatures. For 

 aqueous solutions baked caoutchouc was found to work fairly well. Among 

 the various cements that may prove successful is bakelite. 3 This is briefly 

 described as being made of equal amounts of phenol and formaldehyde to 

 which a small amount of an alkaline condensing agent is added. This 

 latter is a compound made of ammonium carbonate, soap, sodium carbonate, 



1 Carnegie Institution of Washington Publication No. 60. 



2 Ibid., No. 110. 



s Journ. Ind. and Eng. Chem. (March), p. 156 (1909). 



19 



