176 HYDRATES IN AQUEOUS SOLUTION. 



optical paths (except the absorbing liquids) by means of two trains of reflect- 

 ors or lenses suitably placed and adjusted. This plan was given a very 

 thorough test, and was finally abandoned for the one explained below, for 

 the reason that by the above-mentioned scheme it was necessarily trouble- 

 some to maintain two independent trains of reflectors or lenses, or totally 

 reflecting prisms, which would produce in the field of view, when the cells 

 were removed, two emission spectra of exactly equal intensity and also close 

 together. 



A much simpler arrangement, and one which left nothing of a practical 

 nature to be desired, was the following (see fig. 65): Light emitted by 

 the Nernst glower G fell upon the concave speculum reflector R. This 

 reflector was the unruled plate for a concave grating. The reflected beam 

 came to a focus at the slit of the collimator, i. e., the axis of the image of 

 the glower coincided with that of the slit. Since the distance between A 

 and B was 286.5 cm. and the distance between the reflector and the slit was 

 305 cm. (10 feet) the beam of light did not greatly exceed the width of the 

 glower's image for a distance of 3 or 4 cm. on both sides of the precise image. 

 Consequently, when a cell of length less than 3 cm., and so constructed as 

 to contain two solutions separated by a very thin diaphragm, was placed 

 in contact with the slit-jaws, any differences in the spectra of the light trans- 

 mitted by the two liquids were correctly ascribed either to differences in 

 absorptions, or to inequalities in the transparency of the ends of the com- 

 partments of the compound cell. It was possible, of course, to make the 

 halves of the cell so nearly alike as to produce no differential influence on 

 the transmitted beams which could be detected visually. Even if this 

 effective equality in the compartments of the cell could not have been 

 realized, it would have been a comparatively simple matter to have inter- 

 changed the solutions in these compartments, and to have observed the 

 phenomena which remained constant, and which were, therefore, due to 

 the absorption of the liquids and not to the containing-walls. 



The more exact details and dimensions of the cell will now be given. The 

 lower compartment was a box made of five rectangular strips of glass, 

 cemented together. Its interior dimensions were: Length 2.5 cm., width 

 2 cm., and depth 1cm. The upper compartment was made of four vertical 

 glass walls cemented together at the edges, and to a sheet of platinum foil as 

 a bottom. The interior dimensions of this little box were: Length 2.5 cm., 

 width 0.94 cm., and depth 1 cm. The platinum foil was 0.06 mm. thick. 

 Care was taken to make the four end pieces of glass of the same thickness 

 and from the same sheet. These ends were parallel to each other and at 

 right angles both to the bottoms and to the side walls of the respective com- 

 partments. Platinum was used as the partition between the boxes, since it 

 is opaque to visible radiations, and since it is not acted upon chemically by 



