Chap. 29.] Steel. 165 



of plumbago. By folution in acids, it is found that 

 call iron contains a large quantity of plumbago, that 

 fteel contains fome of it, but that the moft malleable 

 forged iron contains fcarcely any ; it therefore appears, 

 that fteel is in an intermediate condition between caft 

 iron and forged iron, and that, in making the former 

 into the latter, the metal muft pafs through the ftate 

 of fteel. In fome foreign works they actually manu- 

 facture fteel by only flopping the procefs of refining 

 at a certain point. In thefe manufactories, however, 

 they make ufe of an ore of uncommon purity ; and 

 the fame procefs does 'not fucceed with ordinary ores, 

 becaufe the metal obtained from thefe contains other 

 impurities befides plumbago, which cannot be fepa- 

 rated without reducing the iron to its malleable 

 ftate. 



There are differences in the working of iron, ac- 

 cording to the ores from which it is obtained, the caufes 

 of all of which have not been difcovered. The pre- 

 fence of phofphoric acid, however, is known to pro- 

 duce a brittlenefs in iron when cold, which occafions 

 that fort of iron to be called cold fhort iron by the 

 workmen. Phofphoric acid is chiefly found in iron 

 obtained from bog ores. 



The workmen employed in tempering fteel judge, 

 by the different colours it afiumes during the opera- 

 tion, of the degree of hardnefs it has acquired. That 

 thefe colours may be properly obferved, fome part of 

 the metal to be tempered mould be fmooth. 



The changes of colour depend on the calcination 

 of the. iron ; for if the contact of air is prevented by 

 the thinneft covering of any oily matter, the effect is 

 entirely prevented. The colour is firft a pale yellow j 

 ;f a piece of fteel is then (truck off, it will be very 

 hard. If the fteel is left in the fire for a longer time, 

 M 3 ic 



