1825.] Philosophical Tramaciiom for 1824, Part II. 223 



When 385 lbs. had been npon the bar about a minute, it emitted 

 a faint creaking sound, and consequently I ceased to add fresh 

 weights ; in about fourteen minutes the bar broke, exactly in the 

 middle of the length. 



" On comparing the fractures of the specimens, there was no 

 apparent difference except in colour. The grain was fine and 

 equal ; the small sparkles of metalhc lustre abundant, and 

 equally diffused ; but in the harder specimen they had a whiter 

 ground. 



'* From these experiments it appears that the elastic force of 

 steel is sensibly the same in all states of temper. 



^' The height of the modulus of elasticity, calculated by the 

 formula you have given in your Nat. Phil. (vol. ii. p. 48) is. 



According to the first experiment 8,827,300 feet. 



And according to the second experiment . . 8,810,000 



" Now the height of the modulus, as you had determined it 

 for steel by Experiments on Vibration, is 8,530,000 feet (Nat. 

 Phil. ii. p. 86). The modulus for cast steel calculated from 

 Duleau's experiments (Essai Theorique et Experimental sur le 

 Per forge, p. 38) is 2,400,000 feet, and for German steel 

 6,600,000 feet. 



" The force which produces permanent alteration is to that 

 which causes fracture in hard steel, as 350 : 5^ ; or as 1 : 1*66 

 in the same steel of a straw yellow temper as 150 : 385, or as 

 1 : 2-56. 



" When the tension of the superficial particles at the strain 

 which causes permanent alteration is calculated by the formula 

 given in my Essay on the Strength of Iron, p. 146, 2nd Edition, 

 it is 45,000 lbs. upon a square inch in tempered steel ; and the 

 absolute cohesion 115,000 lbs. Mr. Rennie found the direct 

 cohesion of blistered steel to be 133,000 lbs. (Philosophical 

 Transactions for 1818.) 



*' But in the very hard bar, the strain which produced perma- 

 nent alteration was 51,000 lbs. for a square inch, and the abso- 

 lute cohesion only 85,000 lbs. 



^' From these comparisons I think it will appear, that in the 

 hardening of steel, the particles are put in a state of tension 

 among themselves, which lessens their power to resist extra- 

 neous force. The amount of this tension should be equal to the 

 difference between the absolute cohesion in different states. 

 Taking Mr. Rennie's experiment as the measure of cohesion in 

 the soft state, it will be 133,000 - 115,000 = 18,000 lbs. for 

 the tension with a straw yellow temper ; and 133,000 — 85,000 

 = 48,000 lbs. for the tension in hard steel. And if this view of 

 the subject be correct, the phenomena of hardening may be 

 explained in this manner, which nearly agrees with what you 

 have observed in your Lectures, I, p. 644 ; after a piece of steel 



