Use a moderately strong solution of chloride of cobalt in water.
"Scientific American, Volume XLIII., No. 25, December 18, 1880" by Various
Meyer and Stopp found the water to contain a solution of chloride of cobalt.
"The Romance of Natural History, Second Series" by Philip Henry Gosse
Stahl had the happy inspiration of making use of the colour-changes of cobalt chloride.
"Rustic Sounds and Other Studies in Literature and Natural History" by Francis Darwin
The salts of cobalt, as the acetate, nitrate, sulphate, and chloride, possess a like property.
"Scientific American, Vol. XXXIX.--No. 24. [New Series.], December 14, 1878" by Various
The print thus obtained is then immersed in a ten per cent solution of chloride of cobalt.
"Photographic Amusements, Ninth Edition" by Walter E. Woodbury and Frank R. Fraprie
Among such substances are solutions of the chlorides of cobalt and of nickel.
"Encyclopaedia Britannica, 11th Edition, Volume 14, Slice 5" by Various