METHOD FOR DETERMINING TRANSPIRING POWER 291 



of the cobalt-chloride paper, as the latter absorbs moisture, 

 comes to have just the same intensity as the color standard. 

 This occurs appreciably before all blueness disappears from the 

 cobalt-chloride slip. A test is considered as ended when the 

 slip just matches the color of this less intense standard. Since 

 the two color standards are permanent in color, they furnish 

 two fixed points in the series of color intensities that are presented 

 as the test paper alters from intense blue to pink, and the test 

 now becomes, simply, to determine the length of time elapsing 

 between the occurrence of a fixed strong intensity of color and 

 that of an equally fixed weak intensity. This modification 

 has been found to decrease the final error, in determining the 

 end point, to a marked degree. 



PREPARATION OF THE PERMANENT COLOR STANDARDS 



The color standards are made by precipitating Prussian 

 blue in filter paper of the same kind as is used for preparing the 

 cobalt-chloride paper. This precipitation is affected by bringing 

 ferric chloride and potassium ferrocyanide together in the paper; 

 if the pigment is formed in the solution outside of the paper 

 it has been found impossible to cause it to enter uniformly after- 

 wards. The paper is first impregnated with ferric chloride, 

 by immersing for the proper time in a solution of this salt, of 

 the proper concentration, and the excess of solution is then re- 

 moved, first by means of the rubber roller alone and finally by 

 means of roller and clean blotting paper. Next, without fur- 

 ther drying, the paper is placed in a properly concentrated solu- 

 tion of potassium ferrocyanide and allowed to remain there for 

 the proper time, after which it is dried with roller, with roller 

 and blotting-paper, and finally with hot electric flatiron and 

 blotting paper. If the solutions have the correct concentration, 

 if the paper is left for the right length of time in each one, if 

 the blotting paper used is clean and fresh for each application, 

 and if numerous other details are properly carried out, the re- 

 sulting paper has the correct quality and intensity for the 

 color standard. 



