L. MECHANICS AND ENGINEERING. 483 



Some kinds of iron exhibit what is known as the passive 

 state, and are unacted upon by acids until this state has 

 been destroyed by heating. The surfaces thus prepared are 

 inclined to rust very soon. After a series of experiments 

 with nitric, sulphuric, and hydrochloric acids, and etching so- 

 lutions of copper salts, Professor Kick found that a mixture 

 of equal parts of hydrochloric acid and water, to which was 

 added a trace of chloride of antimony was the best etching 

 solution. The chloride of antimony seems to render the 

 iron less inclined to rust, so that after washing thoroughly 

 in warm water, and applying a coat of Damar varnish, the 

 etched surface may be preserved quite clean. The smooth 

 surface that is to be etched is surrounded by a ridge of wax 

 an inch high, as is done in etching copper for plates, and the 

 acid is poured into the disk thus formed. At a temperature 

 of 55 to 65 Fahr. the action soon begins, as shown by the 

 gas evolved ; in winter the etching is poor. The time re- 

 quired is from one to two hours, but the etching should go 

 on until the texture is visible. Every half-hour the acid can 

 be poured off without renewing the wax, the carbon rinsed 

 off, and the surface examined. If too much chloride of anti- 

 mony is added to the acid, a black precipitate will soon form, 

 which can easily be distinguished from the carbon. One 

 drop of chloride of antimony to the quart of acid is sufficient. 

 When the etching is finished the wax rim is removed, the 

 iron washed first in water containing a little alkali, then 

 in clean water, brushed, dried, and varnished. If in a few 

 hours it begins to rust, the varnish should be removed with 

 turpentine, which will also take off the rust, and then var- 

 nish again. 



The appearance of different kinds of iron, when etched, is 

 essentially as follows : Soft or sinewy wrought iron of excel- 

 lent quality is attacked so equally by the acid, and so little 

 acid is separated, even after several hours' action, that the 

 surface remains bright and smooth. Fine-grained iron acts 

 the same; the surface is still smoother, but a little darker. 

 Coarse-grained and cold-short iron is attacked much more 

 violently by acid than that just mentioned. In ten minutes, 

 especially with cold-short iron, the surface is black. After 

 thirty minutes a black glass can be washed off, and the surface 

 will remain black in spite of repeated washings, and exhibits 



