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588                    METALLURGY OF CAST IRON.
If It is desired to print from the etching, more care must be taken. The specimen should be perfectly flat,  if possible, with two parallel surfaces. The etching- solution used is weaker  say one nitric acid, and fifty or even one hundred water. A small brush can be used to advantage to run over the top of the specimen to remove the spent acid and keep a good circulation. This makes the etching process slow but uniformly even. The result, however, is really fine, and the novice will do well to practice on wrought iron, which gives beautiful etchings. In printing from these etched specimens an ordinary printer's roller, not too heavily charged with ink, is used, and the paper must be a superfine calendered variety which is perfectly smooth. additions of steel or wrought scrap, the appearance of the etchings is a guide to the probable wearing qualities. The samples must be first prepared by filing or grinding to get a flat surface. Then this is.smoothed with successive grades of emery cloth until a bright surface is obtained which is not too deeply scratched. This polished surface must not be touched with the fingers, as anything of a greasy nature prevents the acid from attacking the iron. Now the piece is immersed face up in nitric acid diluted with ten parts of water. It is best to use this mixture cold. A few seconds will suffice to bring out the structure. The test piece is then taken out and washed thoroughly in running water.