282 Notice respecting Kalive Concrete Boracic Acid. 



It is advisable to draw the hearth at the end of every twelve 

 hoiir», in order that it mav cool ; for a cool hearth works 

 pleasanier, and makes better produce than one which has 

 been suffered to heat. l"he hearth should be drawn about 

 two watchin2;s after throwing on the la>t of the six bings of 

 ore. As soon as the hearth is walched-out the last tniic, 

 the action of the bellows is stopped, and the smelters draw 

 out the whole of the hot brouse with their shovels, and 

 throw it on the floor to cool, picking out such slags as they 

 may observe; they also remove whatever adheres to the 

 sides or back. 



If the liearth has been properly attended, and a due pro- 

 portion of fuel used, it will scarcely appear hotter in one part 

 than another; and, if it has been working with a free ore, 

 should not appear hotter than a very dull obscure red heat. 



With a free ore, the hearth, when fresh set-up, works as 

 described page 260, iheblast finding its way equally through 

 all parts of the breast. The brouse, when watchcd-out, is 

 dry, and mostly in small pieces, the slags firm, and easily 

 distinguished bv their cavernous appearance and brighter 

 colour, and the lead flows from the hearth scarcely red hot. 

 Lead ore, which contains much silver or copper, or which 

 has not been properly cleared from thegangue with which it 

 is mixed in the vein, requires particular attention on the part 

 of the smelter: instead of working dry and open, it be- 

 comes soft and pasty ; the slag, instead of separating in firm 

 pieces, is diffused through the whole like a half-melted 

 scoria, and the least inattention to the fire will set the whole 

 contents of the hearth into a solid mass, or cause it to boil 

 and flow down in a liquid state on the work -stone — the lead 

 flows very hot, and the hearth appears hot and foul. The 

 addition of lime is necessary to correct this defV.et in the 

 ore, which combining with the fluid scoria, solidifies, and 

 thus assists its collecting in masses: care should be taken 

 not to add more lime than is absolutely necessary for the 

 purpose intended, as all extraneous matter thrown in with 

 the ore lessens the produce of lead, 



[i^o be continued.] 



LI, Notice respecting Native Concrete Boracic Jcid. Ey 

 Smithson Tennant, Bscj. F.R.S.&'c. Cvmmunicated 

 ly L, HoRNEH, Esq. Sec. oj the Geological Society*. 



J- Hh boracic acid is not found, like the greater number of 

 substances, in almost every country; but, as far as our present 



• From tlie Transactions of the Geological Society, vol. i. 



knowledge 



