448 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



tain. Lime, however, is not very refractory in contact witli 

 silica. A principle of general application in the employment of 

 fluxes, enables it to be made very useful. Calcareous sandstones 

 are protected in iron furnaces by lime in the flux; excess of 

 litharge preserves lime cupels from the action of glass of lead. 

 Chemical afiinity determines combination both within the crucible 

 and with the materials of it, as in ordinary solutions wnth acids, 

 yet bodies do not bear to one another the same affinities at differ- 

 ent heats; thus, sodium is reduced by iron at a white heat, 

 reduces oxide of iron at a red heat ; is reduced by carbonic 

 oxide at a cherry-red heat, and reduces it at a dull red — the last 

 fact we owe to Deville. The fusible monosilicate of iron pene- 

 trates clays at a moderate heat ; while with more silex, at a white 

 heat, and in quantity not exceeding eight per cent, it combines 

 chemically with silicate of alumina, forming a double salt, and, in 

 Potter's language, "a glaze." This is what occurs in the Stafibrd- 

 shire bricks, which, with one exception, .the vitrified bricks of 

 Walts are, perhaps, those made at the highest heat, and are the 

 leaet permeable, they are blueish in color. 



The material of chief interest in brick is alumina, an element 

 of epicene gender. It is now a base in clay, and again plays the 

 part of acid in aluminate of lime. The first of these is nearly as 

 old as any hills, and almost as varied in its properties ; the last, 

 is a modern fabrication, and pretty definite, it will resist any 

 heat short of that of fusion of platinum. 



Alumina combines with silica in other proportions than those of 

 clay. Stourbridge clay, which has at least the most refractory 

 reputation, will yield at a lower heat than alumina, with half an 

 equivalent less of silex. Such a material was analyzed by 

 Thomson many years ago, and it was said to have been brought 

 from Chester, in Delaware. Clays vary much in nature, being 

 soft and plastic in decomposed felspars ; gritty in kaolin; hard 

 in marl and slates, these are fire-proof in proportion to the 

 excess of alumina. The}' vary in contraction, by dr3-ing, from 

 one to fifteen per cent. The temperature for burning varies from 

 ten degrees wedgwood in London bricks, to one hundred and 

 twenty for Welsh bricks. Stourbridge and Stafi"ordshire bricks 

 are burnt at eighty -five and ninety-five degrees respectively. — 

 Prideaux speaks of crucibles almost entirely made of silex as 

 very successful ; great pressure was used in their manufacture 



