484 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



numerous little moles. Certain parts of the iron are usually 

 eaten deeper, while others, although black and porous", offer 

 more resistance. By allowing the acid to act for an hour or 

 so, then washing, drying, and polishing with a file, a distinct 

 picture is obtained. Malleable cast iron, we know, rusts more 

 easily than wrought iron, and it is an interesting fact that 

 the action of acids is also violent, the surface being attacked 

 very intensely. Gray pig-iron acts like steel; the etched 

 surfaces have quite a uniform gray color. In puddled steel 

 the color of the etching and washing is gray, with a uniform 

 shade, and the lines are scarcely visible. Cement steel has a 

 very similar appearance, the lines being very weak. In Bes- 

 semer and cast steel the surfaces etched are of a perfectly 

 uniform gray color, with few, if any, uneven places. The 

 softer the steel the lighter the color. On etching, the finest 

 hair-like fractures are rendered prominent. A piece of steel, 

 which looked perfect before etching, afterward exhibited a 

 hair-like fracture throughout its whole length. When differ- 

 ent kinds of iron are mixed, the acid attacks that for which 

 it has the greater affinity, while the other is less acted upon 

 than if it were alone. Etching is exceedingly valuable to all 

 who deal largely in iron, as it enables them to determine 

 with comparative accuracy the method of preparing the iron, 

 as in the case of rails, etc., as well as the kinds employed. 

 3 A, October 4, 1874, 523. 



SCHAIITz's REVOLVING FURNACE-BARS. 



The London Iron speaks with much approbation of 

 Schmitz's Revolving Furnace-bars, in which the ordinary 

 straight fire bars are replaced, singly or in pairs, by hollow 

 cylindrical bars, pierced with holes, and so arranged as to 

 be easily capable of revolution. These bars rest on supports 

 which are themselves cylindrical and hollow, and are sup- 

 ported lengthwise by a plate beneath the door of the fire- 

 box, and fitting into a neck made at the near end of the bar. 

 For revolving them a winch is inserted in the hexagonal 

 opening in the front end of the bars, by which they are 

 turned. 



The lighting of the furnace is performed in the ordinary 

 way, and the furnace door can be kept completely closed, 

 the perforations of the hollow bars supplying as much air as 



