14 Studies on Solution. 



of the instrument, due to temperature changes in the air of the 

 room, it was encased in a large box surrounded with an excelsior 

 packing. When the room temperature was kept fairly constant, the 

 drift was negligible. Occasionally readings were taken in the presence 

 of a slight drift, and in this case the zero was redetermined after each 

 deflection and one-half the drift added to the observed deflection. 

 The deflections of the radiomicrometer were observed on a ground- 

 glass scale at a distance of 5 meters. This scale was placed on the 

 table on which was mounted the Nernst glower and cells. This 

 arrangement enabled a single observer to carry out all the measure- 

 ments, i. e., to manipulate the cells, to watch the glower current, and 

 to read the deflections. 



The cells, which were made by Shaeffer 1 and described in his paper, 

 were used in the present work on a few salts only. These cells, which 

 were of brass, gold-plated, and of adjustable depth, although perfectly 

 workable, were found to be somewhat clumsy for this investiga- 

 tion. A cell was required which could be easily and quickly opened, 

 cleaned, and filled. The form of cell finally chosen was very satis- 

 factory. This cell (fig. 2) consisted simply of a glass ring, 4.2 cm. in 

 diameter, closed on each end by a plane-par- 

 allel plate of glass 2 mm. thick. The glass 

 ring was ground to a uniform thickness within 



<- 4. a cms. -* 



0.001 inch. It was found unnecessary to FlG 2 . -Cross-section of cell. 

 cement the glass plates on the glass ring. 



To fill the cell the glass ring was placed on the bottom plate, the 

 solutions poured in, and the upper plate slid on. In the case of 

 water solutions, the cell thus filled was quite tight and remained 

 free from bubbles for several hours; in the case of solutions of methyl 

 alcohol small bubbles appeared in about half an hour. It was some- 

 times convenient to seal the bottom plate on to the glass ring with 

 rubber cement. Six cells were made varying in thickness from 1.844 

 to 21.996 mm. A thick cell K' and a thin cell K (fig. 1), were held 

 in a frame (not shown in fig. 1) and either in turn could be quickly 

 interposed in the path of the light. 



PROCEDURE. 



The solution for which a was to be determined was placed in two 

 cells exactly alike, except that one was thin and the other thick. The 

 energy / of the monochromatic beam of light after passing through 

 the thin cell containing a thickness h of solution, and the energy /' 

 after passing through the thick cell containing a thickness h' of solution, 

 were measured in arbitrary units i. e., deflections of the radiomicrom- 



'C:irno K io Inst. Wash. Puh. No. 230, p. 50. 



