DEPARTMENT OF BOTANICAL RESEARCH. 85 



that of the sample containing the same indicator, both being examined 

 at an arbitrary but equal depth, the latter also being made equal to the 

 depth of the shield solutions employed. 



The use of shield solutions necessitates employing a colorimeter 

 with hollow cylindrical plungers which screw off conveniently. If (a) 

 denotes the right-hand plunger and cup and (6) the corresponding left- 

 hand parts, the following procedure is recommended: The shield 

 solutions are (A) the pigmented sample and (B) distilled water. As 

 neither of these is changed during an examination of any particular 

 sample, even though different indicators are employed, a measured 

 quantity of (A) is placed in plunger (a), and the same quantity of dis- 

 tilled water in plunger (6) . The amounts to be used should be inversely 

 related to the intensity of color in the sample, and the depth of the 

 column is calculated after determining the capacity of the plunger- 

 tube and its length, since the quantity introduced is also known, thus 

 furnishing three known quantities in the proportion. 



Into the colorimeter cup (6) is then placed the sample plus indicator, 

 and into cup (a) that standard solution (plus indicator) which may 

 seem more or less to correspond in color. The cups are then set by 

 means of the thumb-screws at a depth corresponding to that of the 

 shield solutions employed, and examination is made. It is convenient 

 to arrange a series of serological test-tubes, each with 2 to 5 c.c. of the 

 standard solutions plus indicator, a series for each indicator, differing 

 in value by one-tenth or two-tenths of one unit of the exponential 

 scale. The standard solution in cup (a) is of course changed if the 

 color does not match, and trial is made until there is exact agreement. 



In the microcolorimeter of Duboscq employed, the plunger-tube is 

 usually half-filled with the shield solution, corresponding to 0.625 c.c, 

 and to a depth of 16.5 mm. The amounts of the solutions placed in the 

 cups is immaterial, so long as the plunger touches the liquid, inasmuch 

 as the depth of column examined must be set to correspond with the 

 shield. Moreover, the form of the cup is such that the expanded upper 

 part holds any surplus liquid forced out of place by the plunger. The 

 red indicators (red-yellow or yellow-red) have been particularly serv- 

 iceable, and with the colorimeter the full value of the entire range of 

 the indicator change may be realized. Where indicators overlap in 

 pH the choice would be of the one contrasting best with the sample. 



Soil Aeration Experiments with Helium, hy W. A. Cannon and E. E. Free. 



In previous experiments on soil aeration,^ nitrogen has been used as 

 an inert gas to replace the normal atmosphere of the soil. This pro- 



1 Cannon, W. A. Carnegie Inst. Wash. Year Book for 1915, pp. 63-64; Year Book for 1916, 

 pp. 74r-75; Year Book for 1918, pp. 81-83. 

 Free, E. E., and B. E. Livingston. Year Book for 1915, pp. 60-61. 

 Livingston, B. E., and E. E. Free. Year Book for 1916, p. 78. 

 Cannon, W. A. Amer. Jour. Bot., vol. 2, pp. 211-224 (1915). 

 Cannon, W. A., and E. E. Free. Science (n. s.), vol. 45, pp. 178-180 (1917). 



