Proceedings of the Polytechi^ic Association. 963 



in lead, black lead and otlier substances will answer a good purpose 

 to coat the steel with ; but when a common tire is used, prussiate of 

 potash is to be preferred. The prussiate, powdered fine, may be 

 sprinkled or sifted uniformly over the surface of the article. It may 

 be tlien put into the fire, heated to the pro})oi' degree, and tlien 

 plunged into the water, except when tlie piece of metal is large, in 

 which case a second coating of the prussiate, and a second heating 

 before cooling may be proper. The various solutions that have been 

 tried do not appear to have any chemical action whatever. 



Mr. Xornuin AViard said he believed he was the first to advance 

 the theory of hardening steel, and he was proud that he had done 

 so. When a piece of steel is tempered, it is simply in a state of ten- 

 sion, that is one part of it is trying to get away from the other. If 

 one ])art is contracted in advance of the other, it is contracted with 

 a tensile force similar to drawing out the parts of a truss bridge. If 

 a piece of steel is contracted around the surface, every part will be 

 put in the same tension, each atom tending to pull away from the 

 other. He heated a piece of steel the thickness of a sheet of paper, 

 and placed and cooled it suddenly and eLjually with two wet sponges ; 

 but it was not hardened, and bent easily, and was as soft as brass. If 

 the thick end of a planing knife is first immersed in the water it will 

 become equally hardened without warping ; this will also obviate the 

 cracking usually found in tempering the knives for planing machines. 

 The tempering of steel is simply putting it in a state of tension. 

 The chemical combination of waters for tempering steel can have 

 no eftect on the process. In hardening steel it should be [)Ut straight 

 down into the water without moving it sideways, for the least pres- 

 sure in any direction will cause the side pressed against to be cooled 

 quicker than the other. The specific heat of mercury is .03332, water 

 being 1, This latter substance is a bad conductor of heat. Mercury 

 absorbs heat quite rapidly, and for this reason is used much in the 

 process of hardening steel. The Danuiscus sword blades are tem- 

 pered when the cold north wind blows; wlien taken from the fire 

 the steel is cooled by exposing it to the cold wind which tempers it 

 slowly and evenly. Liquified carbom'c acid gives a temperature of 

 200 degrees below the freezing point ; when used for hardening 

 would make the surface of the steel very hard. 



Dr. Yanderweyde remarked that if iron could be hardened on 

 the surface it would make good magnets. An iron foundery in 

 Twelfth street, near Avenue C, makes very good iron, which is well 



