288 BURTON E. LIVINGSTON AND EDITH B. SHREVE 



PREPARATION OF THE COBALT-CHLORIDE PAPER 



The usual procedure in the preparation of cobalt chloride 

 paper, simply dipping it in the salt solution and then suspend- 

 ing to dry in the room or drying in an oven on a glass plate, 

 has proved unsatisfactory; this procedure gives papers that are 

 not at all uniform, and uniformity of paper is greatly to be desired. 



Munktell's Swedish filter paper, number 00, was recommended 

 by Bakke for this purpose, but only a small percentage of the 

 sheets, as furnished by dealers, can be used, most of them vary- 

 ing too much in thickness. Whatman filters no. 30 (11 cm. 

 circles) have been finally adopted, this proving to be somewhat 

 better than the other, in our tests. 



The cobalt chloride solution used contains 3 grams of the 

 salt (dried at 110°C.) in 100 cc, and is made distinctly acid 

 by addition of a little hydrochloric acid. The dry paper is 

 placed in the solution and the vessel is agitated for one minute; 

 during this process the sheet is turned several times. It is 

 then placed on a clean glass or ferrotype plate and the excess 

 of solution is squeezed out with a photographer's squeegee 

 roller, of rubber. Then a piece of clean, dry blotting paper 

 is laid upon it and the rolUng is repeated, with considerable 

 pressure. Next the circle of paper is partly dried in an oven, 

 between two pieces of blotting paper, but it must not be dried 

 to the blue color. While still pink it is removed from the oven, 

 placed between two sheets of clean filter paper, on a smooth 

 surface, and thoroughly dried by heat and pressure, applied 

 by an electric flatiron. When the sheet is thoroughly dry the 

 blue color is compared with the more intense of the two color- 

 standards to be described below. To be acceptable, the blue 

 of the cobalt chloride paper must be distinctly more intense than 

 that of the standard. The intensity of color may be varied by 

 altering the length of the time period during which the paper 

 remains in the solution, which seems more satisfactory than 

 changing the concentration of the solution. 



