BURNING BRICKS. 381 



which there appears no indication of the pre- 

 fence of this nictai. Perhaps the true rcafon 

 is to be looked for in the fund intermixed, which 

 may he often fufpccled to contain many par- 

 ticles of fcintillating fpar. I fufpcdl alfo, 

 that it fometimes participates of the nature of 

 gypfum or fluor mineral. One thing certain, 

 is, that vitriolic acid is ahnoft always intermix- 

 ed with the clay : and hence the fulphureous 

 fmell that is always felt in the neighbourhood 

 of brick-kilns. And, if lime be intermixed 

 with the clay, it mud unavoidably abforb the 

 acid : for which rcafon an examination with 

 acids will always be found fallacious. 



If the iron be combined with vitriolic acid, 

 the colour appears in the burning. For as the 

 violence of the fire incrcafes, it aflumcs firft a 

 yellow colour, then a red, then a dark grey, 

 and at length a deep black. The colour is 

 darker or lighter in proportion as the quantity 

 of iron intermixed is greater or lefs. If the 

 burning docs not expel the whole of the vitrio- 

 lic acid, the acid often attracts moifture from 

 the air, and cfllorcfccs in the form of alum. 



5 vi. How Bricks JJjould be formed and burnt. 



HRICKS confiil univcrfally of clay and filice- 

 ous earth. The clay renders the mafs duclilr, 

 f ind fufceptiblc of induration. But as clay, by 



itJelf. 



