Those who occasionally print by the well-known fer-ro-prussiate process, as well as engineers who copy tracings by this means, may make a note of a rapid and easy method of writing details on such prints. A solution of carbonate of soda or caustic soda, frequently recommended for the purpose, is not nearly so good as one of potassium oxalate. The uniform strength of solution is important, though why a variation in the strength of such a neutral substance as potassium oxalate should make any difference I am unable to say, However, 75 grains dissolved in an ounce of water will remove the blue ground of the drawing in a few seconds, and can be applied with a pen or fine brush, the solution, if necessary, being thickened with gum. The paper should be well washed afterwards, for, if this is not done the blue color is very likely to reappear. Engineers who use this method on large tracings frequently content themselves with mopping off the surplus solution with blotting-paper and " washing " the treated part by applying wet blotting-paper once or twice. This imperfect method of removing the chem-cals is, no doubt, responsible for the complaint made in many engineering shops that details written in this way gradually disappear from the drawing, the blue ground being gradually restored. - Photo-American.