292 BURTON E. LIVINGSTON AND EDITH B. SHREVE 



Since ferric chloride is extremely hydroscopic and since it 

 cannot be dried to constant weight without undergoing decom- 

 position, it is impossible to give explicit directions for preparing 

 the solutions of this salt. The proper concentration is to be 

 obtained by means of preliminary tests (in the actual prepara- 

 tion of filter paper circles). This concentration might have 

 been determined by analysis, but this was thought unnecessary, 

 since practical tests must be resorted to in any event. The 

 ferric chloride solution used by us for the more intense color 

 standard contained about 2.28 g. of the undried salt (of un- 

 known water-content) in 100 cc. The solution used for the less 

 intense standard contained about 0.13 g. in 100 cc. These solu- 

 tions must be made distinctly acid, by addition of hydrochloric 

 acid. 



Since potassium ferrocyanide may be dried to constant weight, 

 the concentrations used for our solutions of this salt may be 

 definitely stated. The solution employed in preparing the more 

 intense color standard contained 36.8 mgm. of dry salt in 100 

 cc, and the one for the less intense standard contained 5.3 mgm. 

 in 100 cc. It is needless to remark that all the concentration 

 actually used, for both salts, were obtained by diluting much 

 more concentrated solutions. 



The procedure followed is somewhat complicated and the 

 operator requires much practice before perfect pieces of the 

 permanently blue paper may be produced with assurance. The 

 paper circle is first placed in distilled water for one minute, 

 being reversed once. Then it is squeezed by means of the roller 

 on a ferrotype plate. The roller is passed over it in two direc- 

 tions, at right angles to each other. Next, a fresh piece of blot- 

 ting-paper is laid over the circle as it rests on the plate, and the 

 double rolling is repeated, the blotting-paper being renewed 

 for the second rolling. The uniformly moist sheet thus ob- 

 tained is now placed in the proper solution of ferric chloride, 

 where it remains for from one to two minutes (more intense stand- 

 ard) or from ten to fifteen seconds (less intense standard), accord- 

 ing to the temperature. (At 20°C. one minute was sufficient, 

 at 16°C. two minutes were required, for the more intense stand- 



