62 Laboratory Arts 



constructed furnaces, but small jobs, such as small bearings, 

 may be case hardened with sufficient precision for laboratory 

 purposes in a very simple way. Case-hardening simply consists 

 of altering the composition of a forged iron surface giving a 

 very hard and wear-resisting " skin." This is brought about 

 by the addition of carbon. An article to be case-hardened 

 is heated to redness in the ordinary fire and immersed 

 momentarily in a pot containing powdered yellow prussiate 

 of potash, or sprinkled with the prussiate. This becomes 

 melted round the red-hot metal, and in cooling re-solidifies 

 and is chipped or washed off, the process being repeated 

 according to the depth to which hardening is desired. The 

 iron is finally heated to a full red heat, and quenched in 

 water, when it will be found to have acquired remarkable 

 hardness on the surface, without being so liable to crack as 

 a steel forging would be. Wrought iron, forged, and case- 

 hardened in this way, may be used for the making of lathe and 

 other tools in cases of emergency. 



Silver Soldering. For this process, which is used in 

 cases where a very strong joint is required, proceed exactly as 

 for soft soldering, so far as the cleaning of the surfaces is con- 

 cerned. Then place in position and cover the joint with 

 powdered borax and heat till the first action ceases and the 

 borax melts quietly. 



Sprinkle over this a few filings of " silver solder " (2 parts 

 standard silver, i part brass wire, melted together) and heat with 

 a small blowpipe flame, till the metal " runs " into the joint. If 

 more solder be required it should be added in the form of 

 a cream of solder filings, powdered borax-glass, and water, 

 and heated up again as before. 



If the joint is required to be very hard, as well as strong, the 

 silver solder employed should be composed of three parts by 

 weight silver and one part brass. 



Tempering Steel. The tempering of a steel chisel has 

 already been dealt with, but it is necessary to give rather more 

 detailed instructions in order to cover all the examples that 

 may be met with in laboratory work. 



It is not always possible to make use of the most direct 



