298 BURTON E. LIVINGSTON AND EDITH B. SHREVE 



ent types of this apparatus for the field have been devised, but 

 they all agree in the absence of any means for determining the 

 temperature of the evaporating surface, or even of the water 

 behind that surface. Perhaps the most convenient form of 

 evaporating-surface apparatus is that devised by Bakke,^ which 

 is a much improved form of the original Harvey instrument.^ 

 We understand that this type of instrument is to be placed 

 upon the market. 



A somewhat less convenient but less expensive form of this 

 apparatus, has been developed in our own work. It is a simple 

 modification of the laboratory form above described. A small 

 porous clay cup (about 2 cm. in diameter and 5 or 6 cm. long) 

 is set into a flat cork stopper just as the larger one is set into a 

 block of wood, the top of the cup being ground flat to coincide 

 with the upper surface of the stopper. Over this surface is 

 cemented a piece of hard rubber 1 mm. thick, with an opening 

 over the porous plate, as in the other form. This rubber plate 

 is trimmed at its margins to the size of the stopper. The latter 

 fits a large-mouth bottle of squat form (to avoid its being upset 

 too readily) and the porous cup extends into the bottle nearly 

 to the bottom. The bottle is nearly filled with distilled water, 

 the cup and stopper are put into place, and the whole is then 

 inverted. This expels the air from the cup, and when the in- 

 strument is again placed upright the cup remains filled with 

 water, as in the larger form. Aside from the fundamental ob- 

 jection arising from somewhat uncertain temperatures of the 

 evaporating surface, this type of instrument has proved very 

 satisfactory. 



TEMPERATURE RELATIONS OF RATE OF COLOR CHANGE IN 



COBALT-CHLORIDE PAPER 



As has been well pointed out by Bakke, the only condition 

 outside of the hygrometric paper itself, that can be influential 

 in determining the time period required for its color change 



8 Bakke, A. L., and B. E. Livingston, Further studies on foliar transpiring 

 power of plants. Physiol. Res. 2: 51-71, 1916. 

 3 Livingston, 1913, I.e. 



