4fc On the Affinities of 



III. 100 grains of the same clay were exposed 'with two- 

 grains of lamp carbon. The latter disappeared, and a par^ 

 tially vitrified mass of clay was obtained of a blackish blue 

 colour. 



IV. 100 grains of clay and 3 grains of carbon afforded a 

 result similar, though without any symptoms of vitrification. 

 A portion of carbon equal to half a grain remained untaken 

 up, 



V. 100 grains of clay were exposed in a Cornwall clay 

 crucible along with 8 grains of carbon, to a heat of 1 70? 

 of Wedgwood. The result was a mass more friable than 

 any of the former. Its colour was black, and the interior of 

 the crucible was covered with a dark lead blue glaze, which 

 had penetrated a little way into the thickness. , As was ex- 

 pected, a considerable quantity of the carbon remained un- 

 taken up. It is presumable, therefore, from these experi- 

 Hients, that the addition of charcoal, in quantity not exceed- 

 ing l-50th or !-40th the weight of the clay, hastens its 

 fusibility by uniting with it, but that beyond' this propor- 

 tion the fusibility of the mixture is retarded. 



In the fabrication of crucibles the same fact is clearly 

 illustrated. Cast steel pots are frequently made with pow^ 

 dered black lead pots mixed with the Sturbridge clay. This 

 lenders them more fusible, but less apt to clink in heating 

 and cooling. An extra dose of either black lead or coke dust 

 in forming smaller crucibles decreases the fusibility, but by 

 impairing the tenacity of the compound in high heats en- 

 tails an evil of as great importance as that meant to be re- 

 moved. 



VI. A Sturbridge clay crucible filled with charcoal, upon 

 which a lid was accurately fitted, was exposed to a heat of 

 sufficient strength to melt it into a flattish cake. In this 

 state the consistency was soft, and nearly entering into fu- 

 sion. When cold, I found a great portion of the charcoal 

 still inclosed. The interior of the pot was of a rusty black 

 colour, and honey-combed. The fracture was porous, 

 throughout, and exhibited an imperfect glassy appearance 

 of a dirty black colour. The fracture of vitrified Sturbridge 

 clay in general is white, and even when urged to fusion is 

 of a dull yellowish colour. The result now obtained was 

 different from the common appearances, evidently in con- 

 sequence of the quantity of charcoal inclosed, and readily 

 presented to the clay at a high temperature. 



VII. Fifty grains of SturbYidge clay, mixed with 3 grains 

 of chalk, or nearly 1-1 7th of its" weight, were melted into a 



whitish 



