EXPERIMENTAL METHODS. 21 



This apparatus was found to work very well for aqueous solutions. 

 Some evaporation took place at the higher temperatures, but distilled 

 water was added in proper quantity and mixed with the solution so as to 

 keep the concentration constant. By using troughs of different lengths it 

 was possible to vary the length of salt solution through which the light 

 beam passed from 1 to 200 mm. One inconvenience was experienced at 

 low temperatures; moisture would sometimes condense upon the exposed 

 prism-faces. To overcome this an air-blast was directed upon these faces, 

 and this helped very materially to prevent the condensation of moisture. 



For the investigation of glycerol and other solutions a cell made of 

 fused silica was used. The cell, as received from the Silica Syndicate Com- 

 pany, did not have plane parallel ends. At the suggestion of Dr. Pfund 

 these were ground down with finely powdered emery and rouge. In this 

 way a very serviceable cell was obtained. The depth of liquid in the cell 

 was 104 mm. 



For work on the effect of high temperatures on absorption spectra, 

 a closed steel cell is being made. This is intended to stand the pressures 

 exerted by the alcohols, acetone, ether, water, etc., at their critical tem- 

 peratures. The ends contain quartz windows and the whole interior of 

 the cell will be lined with gold. At the same time a radiomicrometer is 

 being made and an apparatus is being devised by means of which quanti- 

 tative measurements of the energy absorption for all parts of the spectrum 

 can be obtained. 



In the case of solids, the time of exposure is necessarily long, usually 

 occupying several hours. It is very necessary in this case to screen off 

 stray light. The method is very simple, consisting in focusing by means 

 of mirrors or lenses the light from a Nernst glower or an arc upon the salt. 

 The salt is placed a short distance from the slit of the spectroscope, so that 

 the directly reflected light does not enter the slit. By this means only the 

 diffusely reflected light enters the slit, and in general this light has pene- 

 trated somewhat into the salt and, accordingly, some wave-lengths are 

 partly or wholly absorbed. The salt is placed at such a distance that the 

 grating is completely filled with the beam of diffusely reflected light enter- 

 ing the slit. This method has been used by several investigators, notably 

 by Anderson ' and Schultz. 2 



The observations on the phosphorescence of uranium compounds 

 were made with the Hilger spectroscope, from which wave-lengths can be 

 read directly. Either sunlight or the light from a spark was used as the 

 source of light by means of which the phosphorescence could be excited. 

 Screens of variously colored glasses were used in order to find whether the 

 wave-length of the exciting light had any effect upon the phosphorescent 

 spectrum. Especially valuable was a glass screen 3 that absorbed all wave- 



1 Astrophys. Journ., 26, 73 (1907). 



2 Diss., Johns Hopkins University, June (1908). 



3 This glass screen is very useful for observing Haidinger's brushes. These brushes 

 as seen by the naked eye are yellow and purple. Using a blue-glass screen the yellow 

 fringes become dark and show no color. With the above-mentioned screen the brushes 

 are red. These facts corroborate the theory given by G. G. Stokes: Collected Papers, 

 2, p. 362. 



