If fracture gives long silky fibres of leaden-gray hue, fibres cohering and twisting together before breaking, may be considered a tough, soft iron. A medium even grain mixed with fibres, a good sign. A short, blackish fibre indicates badly-refined iron. A very fine grain denotes a hard, steely iron, apt to be cold, short, hard to work with the file. Coarse grain with brilliant crystallized fracture, yellow or brown spots, denotes a brittle iron, cold, short, working easily when heated; welds easily. Cracks on the edge of bars, sign of hot, short iron. Good iron is readily heated, soft under the hammer, and throws out but few sparks.

All iron contains more or less carbon -- the hardest the most.

The breaking strain on various metals is shown in the following table, the size of the rod tested being in each case one inch square, and the number of pounds the actual breaking strain:

Hard steel,.........150,000 pounds

Soft steel..........120,000

Best Swedish iron,.....84,000

Ordinary bar iron,......70,000

Silver,...........41,000

Copper,..........35,000

Gold,...........22,000

Tin,............' 5.500

Zinc............ 2,600

Lead,........... 860