( 168 ) 



should be different. As the solutions get more diluted, they exhibit 

 colours approaching the shade of the colour-acid. 



A very striking example, which lends itself well for practical 

 demonstration, is furnished by sodium carminate. Sodium alizarate 

 may be also used, only the solutions, on being diluted, soon become 

 turbid, owing to the slight solubility of alizarine. 



From these examples it follows that the fundamental principle 

 of colorimetry ought to be expressed as follows: 



Solutions of the same colouring matter, when tested colorimetrically , 

 exhibit in layers of the same thickness the same intensity oj colour if 

 they i^ossess the same concentration. 



The colorimeter. 



This apparatus must be so constructed that the liquid under exa- 

 mination may be brought to practically the same concentration as 

 the standard liquid. 



The apparatus best suited for this purpose is that of Salleron ^), 

 modified by Koppeschaar '). In this colorimeter the most concen- 

 trated of tiie two solutions is diluted with water until it has the 

 same colour as the weaker solution. Fromjhe amount of water added, 

 the desired concentration is calculated. It is a matter of indifference 

 whether the most concentrated or the most diluted solution is used 

 as the standard liquid. 



As it is not possible, when using the colorimeter of Salleron- 

 Koppeschaar, to make rapidly successive readings of a quantity of 

 solution to be measured, the apparatus, which I am now using and 

 which is represented in the annexed drawing, is perhaps to be pre- 

 ferred. It is constructed from a colorimeter of C. H. Wolff '). The 

 tube containing the standard liquid, the standard ^^i^e >S is connected 

 by means of a small horizontal tube E with the glass cylinder A 

 in which a plunger C is suspended. This plunger can be moved up 

 and down by means of a cog-wheel device along the standard B. Jn 

 this loay the level of the standard solution may be raised or loivered : 

 by providing B with a scale, the position of the liquid may be read 

 off on the same. 



The actual colorimeter stands in the dark chamber D. It consists 

 of the standard tube S and the tube containing the liquid to be 

 measured, the measuring tube M. 



1) Zeitschiift fur Anal. Chemie 11 p. 302. 



2) Zeitschrift fur Anal. Chemie 38 p. 8. 



3) Dingl. 23G. 71. 



