PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 447 



night-boat, fire from the smoke-pipe would not be visible, a 

 blower being used as an exhausting source for draft. 



Mr. Nash remarked that formerly it was considered impossible 

 for ships to enter a harbor defended by stf.tionary batteries 

 without being destroyed. The fallacy of that opinion had 

 been shown at Elsinore, and more recently at Port Royal. 



CRUCIBLES AND FIRE-PROOF MATERIALS. 



Mr. J. H. Churchill read the following paper : 



The subject is of the greatest importance to the owner of 

 glassworks — the manufacturer of steel — the assayer of gold — 

 the worker in brass. Shareholders in gasworks, proprietors of 

 veins of quartz, or of dry river bottoms, of lime kilns, of clay 

 beds, of magnesia quarries, are all interested. • Salt even contri- 

 butes to the general result. 



As one instance of this : A recent report to the Emperor 

 of France estimates the diiference of cost for fire-proof ma- 

 terials in making steel at thirty francs per ton between 

 that country and England. Another, the cost of working 

 ing up a quarter of a ton of platinum for the Russian government, 

 was reduced five hundred fold by a process growing out of the 

 work of M. Deville, by the use of lime cuprels and gaseous fuel. 

 My connection with the subject is this : In contriving some 

 experiments on a subject cognate with that chemist's work on 

 the alkaline metals, I had occasion, two years ago, to examine 

 carefully the principles on which his results were founded. I 

 have since followed many of them back into the hands of metal- 

 lurgists, who believed in alchemy. The best working authori- 

 ties are, perhaps, Schlutter, Kirwan, Chaptal, Berthier, Hasen- 

 fratz and Deville. 



The materials used in making crucibles are, chemically 

 speaking, silica, magnesia, alumina, lime, graphite. They occur 

 in nature, and geologists know them chiefly in combination, and 

 deteriorated, as granites, gneiss, clay, slates, marl, quartz, 

 plumbago, gas carbon. 



Singly or in binary combinations they are very infusible — in 

 ternary combinations with each other, or in presence of other 

 matter, it is often otherwise. 



Steatites, composed chiefly of silicate of magnesia, are more 

 or less fusible in proportion to the quantity of alumnia they con- 



