424 
Cast Steel . 
upon the surface of brightened steel, may serve to indicate 
the want of uniformity in its composition, as well as the 
method by an acid which has before been explained in 
this work ; and second, that the deep colour being obser- 
ved to come on first at the thickest parts, Mr. Stodart was 
disposed to think, that its more speedy appearance was 
owing to those parts not having been hardened. But upon 
trial with a plate of steel made quite hard at one end, and 
left soft at the other, I found that heat applied in the mid- 
dle produced the regular changes at both ends precisely in 
the same manner. 1 suppose, therefore, that the thicker 
parts sinking deeper into the hot metal, experienced a 
stronger re- action and better contact, which may have ac- 
celerated the communication of heat. It may be here no- 
ticed, that we found upon repeating the experiment of 
applying nitrous acid to bright steel, which was hardened 
in part only, the black tinge appeared more speedily and 
strongly upon the hard parts, than the rest of the surface : 
a remarkable event, for the explanation of which I have 
no theory to offer. 
Let us now suppose our cutting instrument to b£ for- 
ged, hardened, and let down or tempered. It remains to 
be ground, polished, and set. The grinding of fine cut- 
lery is performed upon a grindstone of a fine close grit, 
called a Bilson grindstone, and sold at the tool shops in 
London at a moderate price. The cutlers use water, and 
do not seem to know any tiling of the method by tallow. 
The face of the work is rendered finer by subsequent 
grinding upon mahogany cylinders, with emery of differ- 
ent fineness, or upon cylinders faced with hard pewter, call- 
ed laps, which are preferable to those with a wooden face. 
The last polish is given upon a cylinder faced with buff 
leather, to which crocus, or the red oxide of iron is appli- 
ed with water. This last operation is attended with con- 
siderable danger of heating the work, and almost instant- 
