52 Conductivities and Viscosities in Pure and in Mixed Solvents. 



tion of the radiomicrometer. By turning the drumhead of the spectro- 

 scope to the proper points, we could then determine quantitatively the 

 light transmitted by any solution for all wave-lengths of light between 

 A = 0.4,u 



THE DIFFERENTIAL METHOD. 



It was desired to measure the absolute percentage transmission of 

 20 mm. of solution. This was done by a differential method which 

 eliminated corrections for reflection from the glass ends, and differences 

 in the refractive index of the glass and the solutions. The method of 

 procedure is as follows: Cell A is filled with 21 mm. of solution; cell 

 B with 1 mm. of solution. Light of unvarying intensity I was then 

 passed through cell A, and the intensity of the transmitted light I r 

 was measured by means of the radiomicrometer. As soon as possible, 

 cell B was made to occupy the same position formerly occupied by 

 cell A, and the intensity of its transmitted light ! was determined. 

 The deflection produced when cell A was in the path of light, divided by 

 that given when cell B occupied the same position, determines the 

 absolute percentage transmission for 20 mm. of solution; or, in other 

 words, I^/In is the value desired. The justification for this procedure 

 is seen from what follows. 



If the depth of solution under investigation is I, the intensity of the 

 incident light I , and that of the transmitted light I, we have the follow- 

 ing relations for depths of solution I' and I" : 



I, = Io e~ kl> (a) 



In = Io e~ kl " (6) 



Dividing a by 6 we have 



!,/! = e* ( '"- n (c) 



But the actual percentage transmission of the same solution of depth 

 I' I" = I is given by 



I/Io = e- k(l '~ n = e~ k(l "- l \ or I : /I n = I/I (d) 



THEORETICAL DISCUSSION. 



The Ostwald 1 theory of indicators, explaining first the cause of color, 

 and second the difference in sensitiveness of various indicators towards 

 different acids and bases, has been found to be inadequate. . Con- 

 cerning the latter, which is by far the most important side of the 

 indicator question, the Ostwald interpretation is substantially correct. 

 But in consideration of well-known relations between the color and 

 structure of organic compounds, and of the researches of Bernthsen, 2 

 Nietzi and Burckhart, 3 Hantzsch, 4 and others, it has been found neces- 



'Lehrbuch der all. Chem., 1, 799 (1891). 



2 Chem. Zeit., 1956 (1892) ; also Friedlander: Ber. d. deutsch. chem. Gesell., 26, 172, 2258 (1S93). 



3 Ibid., 30, 175 (1897). *Ibid., 32, 583, 3085 (1899). 



