2G2 Mr. Mallet 07i the Physical Conditions 



The fact lias been long known to workers in iron, that no iron, however 

 good and fibrous, will bear being bent double by the hammer, under blows 

 exceeding a certain amount of velocity, known by tact and experience; and that 

 by adroit management, in regulating the slowness with which the iron is so 

 bended double, a very inferior fragile iron may be made to simulate, to an 

 unpractised observer, all the external appearance, when bent, of the toughest ; 

 that, in fact, the rate at which iron can be bent double (cold, of course), is 

 greater in proportion to its original toughness ; but no explanation has ever 

 been offered as to the cause ; and as bad and fragile iron is alwaj's more or less 

 confusedly crystalline in fracture, no observations were made as to any change 

 in its character, dependent upon the rapidity of the strokes or other forces 

 applied to bend it. 



228. One part of tiie phenomena, however, viz., the relation between the 

 toughness and the possible rapidity of bending without fracture, admitted of 

 solution on well-known principles. If we gently apply the force of the hand trans- 

 versely to a stick of cold sealing- w^ax, and continue the pressure long enough, we 

 shall be able to bend it double. If we leave a lump of cold pitch, upon a flat plate, 

 it slowly changes form and assumes that due to a viscous and imperfect fluid ; 

 but if we let the same stick of sealing-wax, drop from the hand upon a marble 

 floor, or if we throw the lump of pitch against a wall, both are shattered into 

 fragments, which alike break with a vitreous or resinous fracture. Nor is this 

 confined to bodies possessing the great ductility and flexibility of pitch or 

 shell-lac ; for, going to the other extreme of rigidity, we find that even glass, 

 proverbially brittle under the slightest shock, slowly yields and changes its 

 form under a constantly applied force, so that the bulbs of very old thermo- 

 meters, exposed for many years to the pressure of the atmosphere, less that of 

 the included column of quicksilver, become diminished in capacity, as proved 

 by the permanent elevation of the zero of the scale of those made by Sanctorio 

 himself; or, again, that the marble slabs of our ancient mantel-pieces, exposed 

 for years to the constant transverse strain of their own weight, and more 

 expanded on their lower than on their upper sides by the radiating heat of the 

 fire beneath them, — gradually sink down, and become permanently curved, the 

 versed sine often reaching in this rigid material ^ of the length. 



229. In the case of slowly applied pressure the effect upon tlie material is 



