On hardening and tempering Steel, 97 
whole than would be for one fingle article. The requifite 
temper may be known by the following circumftances: 
When fuch a heat is given that the tallow is firft obferved to 
fmoke, it indicates the fame temper as that called a ftraw 
colour: this will reduce the hardnefs but little; but if the 
. heat is continued until the {moke becomes more abundant, 
and of a darker colour, it will be equal to a brown, and 
indicates a temper that may be wrought—that is, which 
may be turned or filed, but with difficulty, and only when a 
mild fort of fteel is employed. If the tallow be heated fo as 
to yield a black fmoke, and {till more abundant, this will de- . 
note a purple temper; and if the fteel is good, it will now 
work more pleafantly, though ftill hard enough to wear well 
in machinery. The next degree may be known by the 
tallow taking fire if a lighted body is prefented to it, but 
yet not fo hot as to continue to burn when the light is with- 
drawn; this would equal a full blue colour. Increafe the 
heat till the tallow continue to burn, being once lighted, 
and this will denote a pale blue: and if the whole of the 
' tallow be allowed to burn away, or to burn dry, as the 
workmen call it, it giveswhat clock-makers moftly ufe for their- 
work. Farther tallow is ufelefs; afmall degree of heat 
more would juft be feen in a dark place, or the loweft 
degree of a red heat: fuch is the temper given to the fprings 
for coaches, &c. Thus I have given a reafon why oil or 
tallow is made ufe of, and given you the parallel decrees of 
temper which by a dry heat. are obferved by the change of 
colour only. The method of hardening in quickfilver is of 
great ufe where a fuperior degree of hardnefs is required ; 
and good fieel fo hardened, when the precautions before 
mentioned are duly attended to, will cut glafs like a diamond, 
and turn or cut other fteel at fo high a temper as to differ 
but little from quite hard.—Perhaps at a future time I may 
give you a method by which this hardeft of fteel may alfo 
be worked with confiderable eafe, and the cafes in which I 
have applied it to advantage. 
Vor, II, H INTEL- 
