[Vol. 6 

 180 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



the micro-colorimeter a quantity as small as 1 cc. of sample may 

 serve both for test solution and for the shield. It is preferable, 

 however, to have not less than 2 cc. for most careful work. 



The standardization of the apparatus for this work is ex- 

 tremely simple. It is merely necessary to know the volume of 

 the plunger tube and its length so that in the determinations it 

 will be possible to place a given volume of solution in the plunger 

 cylinder, and knowing the depth which this will occupy, the in- 

 strument may be set so that a similar depth will be examined in 

 the colorimeter cup. This is important, since in one case, as 

 described later, the pigmented sample is placed in the plunger 

 tube and in the other case in the colorimeter cup. Therefore 

 equal depths of solutions will be examined in both cells of the 

 systems. In the instrument at our disposal the plunger tube 

 is 33 mm. in length and the volume 1.25 cc. Since the tube is 

 cylindrical the volume is proportional to length, so that if .625 cc. 

 of solution is added the column has a depth of 16.5 mm. The 

 depth need not be so great as this, and 0.5 cc. of liquid is sufficient. 

 If the quantity of the solution employed is reduced beyond 

 this point, it is necessary to increase relatively the amount of 

 indicator added. Where the total quantity of the test solution 

 is 2 cc, 1 cc. being employed for the sample and 1 cc. for the 

 shield, we find it desirable to use 1 drop of indicator for the 1-cc. 

 sample. If this proves too highly colored the indicator may, of 

 course, be diluted one-half. 



I find it desirable to arrange the samples, standard, and 

 shields as follows: 



In the left plunger tube place the measured quantity of plain 

 water as shield, and in the colorimeter cup of that side place a quan- 

 tity of the sample or test solution plus indicator which shall give 

 any depth greater than that of the liquid column in the plunger 

 tube. In the right plunger tube place the measured quantity of 

 the sample as shield, and in the right colorimeter cup the standard 

 solution plus indicator. 



With this arrangement it is desirable to make up a few standards 

 covering the range of probability, and then in making the 

 determination it is only necessary to change the solution in the 

 right colorimeter cup until an exact match is obtained. There 



