PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 449 



Gas carbon is recommended to be burned into shape. The 

 fluxes to be used by the metallurgist, the alkalis of the glass- 

 worker, the opposite processes of reduction and scorification, 

 the formation of chlorides at high temperatures, and the manu- 

 facture of cyanides, have such special action in each case that 

 must be resisted, that, with variable materials, it is a tolerably 

 obvious conclusion that they should have special applications. 

 Fire-bricks and crucibles have a property belonging to thera very 

 available in this matter. I have mentioned the glazing proper- 

 ties of iron with silica in Staffordshire bricks ; porcelain cruci- 

 bles are glazed externally; further, as we plaster our houses to 

 suit the eye, we may line our crucibles and fire-bricks to suit our 

 work. The crucibles may be thin — too thin or too fusible to 

 support themselves at the highest temperatures — a lining may 

 be then made to help them. Gas carbon and lime have been 

 thus applied by Deville. A lining may be made to absorb part 

 of the flux or oxide, or will assist the purification of the melted 

 metal. Bone-dust and sand have long been thus used in cupel- 

 ling the precious metals ; lime is now thus used in preparing the 

 most refractory. It may be difficult to cause a small per cent- 

 age of metal to collect in one mass, the polished paste of 

 alumina and alurainate of lime offers no impediment ; charcoal 

 permits an easy collection of the separate globules, and it also 

 preserves the contents from the action of oxygen and of the 

 carbonic acid generated by the fire ; lutes applied externally are 

 also used for the latter purpose. In analysis, a platinum cruci- 

 ble, or the ouside of one of porcelain, has been used to effect a 

 better distribution of the heat. 



The permeability of crucibles varies much. The report quoted 

 states that French steel manufacturers seldom use theirs for more 

 than three pourings, on account of the increased porousness ; with 

 a charge which will yield 30 kilogrammes at the first pouring, 

 they obtain only 27 at the second, and 24 at the third. Even 

 density throughout is partly due to thorough vitrification, but is 

 partly produced by mechanical means in the preparation of the 

 clay. The "ring" which tells the thrifty housekeeper that there 

 is no crack, tells the potter of uniform firing. The mechanical 

 part of the result is obtained b}^ reducing the materials to the 

 same degree of fineness by grinding, pugging and sifting. When 

 this is not attended to, the uneven expansion in heating produces 



[Am. Inst,] CO 



