422 
Cast Steel. 
exceed the point at which mercury boils : and by this con- 
trivance the same advantages are obtained in lowering the 
temperature of an whole instrument, or any number of 
them at once, as have already been stated in favour of my 
method of hardeningo Oil is preferable to the fusible 
mixture for several reasons. It is cheaper ; it admits of 
the work being seen during the immersion by reason of 
its transparency \ and there is no occasion for any contri- 
vance to prevent the work from floating, 
I requested Mr, Stodart to favour me with an account 
of the temperatures at which the several colours make their 
appearance upon hardened steel; in compliance with 
which, he made a series of experiments upon surgeons nee- 
dles hardened, highly polished, and exposed to a gradual 
heat while floating at the surface of the fusible mixture* 
The appearances are as follow : 
No, 1. taken out at 430 of Fahrenheit. This tempe- 
rature leaves the steel in the most excellent state for razors 
and scalpals. The tarnish, or faint yellowish tinge it pro- 
duces, is too evanescent to be observed without compari- 
son with another piece of polished steel. Instruments in 
this state retain their edge much longer than those upon 
which the actual straw colour has been brought, as is the 
common practice. Mr. S. informs me, that 430° is the 
lowest temperature for letting down, and that the lower de- 
grees will not afford a firm edge. 
No. 2 at 440°, and 3 at 450°. These needles differ 
so little in their appearance from No. 1, that it is not easy 
to arrange them with certainty when misplaced. 
No. 4 has the evident tinge which workmen call pale 
straw colour. It was taken out at 460°, and has the usual 
temper of penknives, razors, and other fine edge tools. It is 
much softer than No. 1, as Mr. Stodart assures me, and 
this difference exhibits a valuable proof of the advantages 
of this method of tempering. 
