METHOD FOR DETERMINING TRANSPIRING POWER 295 



THE DRYING OF THE SLIPS 



The composite slips may be dried either by heat or by means 

 of a desiccator. If heat is employed some form of desiccator 

 is necessary to retain them in the dry condition until they are 

 used and to bring them to air temperature in the dry state. It 

 has been found convenient to arrange a number of tiny desic- 

 cators, each to hold a single sUp, or perhaps two, and to open 

 these one after another as the slips go into use. Very small 

 opal-glass salve jars, fitted with rubber stoppers, have been 

 found satisfactory for this purpose. They are nearly filled 

 with anhydrous phosphoric acid, which is covered with a circle 

 of fine wire gauze, the paper slip being placed upon the gauze 

 and the stopper forced in tight. Thus the shps remain in the 

 dry condition indefinitely. 



In laboratory work an electric hot-plate is found valuable 

 for quickly drying the sUps; an inverted electric flat-iron is 

 excellent. A metal plate heated by a very small flame is also 

 suitable. In any case, heating should be controlled so as not 

 to injure the paper. Twenty-four hours of continuous heating 

 even at 100°C. will char the paper enough to spoil the colors, 

 and as this effect is doubtless cumulative it is advisable to heat 

 the slips no longer than the few seconds required to change the 

 cobalt paper to a shade more intensely blue than the more in- 

 tense of the standard colors. 



THE STANDARD EVAPORATING SURFACE 



One of the main difficulties encountered in the operation of 

 hygrometric paper tests has heretofore been related to the ar- 

 rangement of the standard evaporating surface. This apparatus 

 has received considerable attention during the progress of our 

 studies, and many different forms have been devised and dis- 

 carded. The most satisfactory form yet tried by us, will now 

 be described. 



The evaporating surface itself is of porous clay, like that of 



