METHODS OF CONVERTING IRON INTO STEEL.

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<he other species of iron tarnish in the same situation. It firstbecomes of a straw-yellow, then of a higher yellow, next purple,then violet, next red, then deep blue, and at last bright blue.At this period it becomes red-hot, and the colours disappear, atthe same time that the metallic scales, or the black imperfectoxid of iron which is formed incrust its surface. All these differ-ent shades of colour indicate the different tempers the steel hasacquired by the increase of heat. Mr. Stodart has availed himselfof this property to give to surgical, and other cutting instruments,those degrees of temper which their various uses require, andhis attention to this circumstance has rendered his tools so justlycelebrated.

Steel possesses a degree of hardness superior to any othermetal. It affords sparks with flint. It may be hammered outinto much thinner plates than iron, and is more sonorous. It isalso possessed of the greatest elasticity, and is perhaps the mostuseful of all metals.

METHODS OF CONVERTING IRON INTO STEEL.

To form steel, bars of malleable iron are bedded in charcoal'in a close furnace, alternate layers of iron and charcoal beingformed. A strong fire is applied for six or eight days; theprogress of cementation, as this process is termed, is known bydrawing a trial bar from the furnace: if this is sufficientlychanged the fire is extinguished, and the metal left to cool forsome days. This forms blistered steel. It is rendered moreperfect by subjecting it to the operation of the hammer, as- inforging iron ; or it is fused, and cast into small bars, formingwhat is termed cast steel. These operations are performed onmalleable iron; but those kinds of cast iron which contain littleoxigen can be converted into steel by a similar process.

In this operation an increase of weight from -pj^th. to -pt^th.is gained. The more carbon is introduced the more brittle isthe steel. Bergman first showed clearly the presence of this mat-