28 WELLS'S NATURAL PHILOSOrHT. 



For example, the workman, in fashioning and slaaping a steel instrument, 

 requires it to bo soft and flexible ; but in usino; it after it has been constructed, 

 as for the cutting- of stone, wood, etc., it is necessary that it should be hard. 

 Tills is accomplished by making the steel soft by auneahug, and then render- 

 ing it hard by heating and cooling quickly.* 



vrhcn will a 54. WliGQ WG beiid or compress a body so 

 compVesled,"'^ tliat its particlcs are separated beyond a certaia 

 break? limited distance, the force of cohesive attrac- 



' • • • • ( 



tion existing between tliem ceases to act, or is destroyed, 

 and the body falls apart, or breaks. 



55. When the Attraction of Cohesion between 



C.in ■we Tc- 



Etorc the at- the pai'ticles of a substance is once destroyed, 



traction of co- , , -^ . , . " 



ho«ion when it IS generally impossible to restore it. Hav- 



destroycd? . D 7 l 



mg once reduced a mass of wood or stone to 

 powder, we can not make the minute particles cohere 

 again by pushing them into their former position. 



In some instances, however, this can be accomplished by resorting to va- 

 rious expedients. The particles of the metals may be made to again cohere 

 by melting. Two pieces of perfectly smooth plate-glass, or marble, laid upon 

 each other, unite together with such force, that it is impossible to separate 

 thera without breakage. In the manufacture of looking-glass plates, this at- 

 traction between two stnooth surfaces is particularly guarded against. 



* There are many practical illustrations in the arts, of the principle, that the modifica- 

 tions of the attractive force which unites the atoms of solid bodies together, are dependent 

 in a great degree upon the forms, or arrangement of the atoms themselves. If we submit 

 apiece of metal to repeated hammering, or jarring, the atoms, or particles of which it is 

 composed, seem to take on a new arrangement, and the metnl gradually loses all its te- 

 nacity, flcMbility, malleability, and ductility, and becomes brittle. The coppersmith who 

 forms vessels of brass and copper by the hammer alone, can work on them only for a short 

 time before they require annealing ; otherwise they would crack and fly into pieces. 



For this reason, also, a cannon can only be fired a certain number of times before it 

 will burst, and a cannon which has been long in use, although apparently sound, is always 

 condemned and broken up. 



A more important Illustration, and one that more closely affects our interests, is the 

 liability of railroad car-axles and wheels to break from the same cause. A car-aile, after 

 a long lapse of time and use, ia almost certain to break. 



That these phenomena are due to changes l;i the manner of the arrangement and the 

 form of the particles, or atoms, of matter, was conclusively proved by an experiment made 

 a few years since in Franco : — An accident having occurred upon a railroad, by the break- 

 ing of an axle, by which many lives were lost, the attention of scientific men was called 

 to the fact, that the iron composing the axle, when first used, was strong, and capable of 

 standing a test, but after use in locomotion for a certain period, could be broken by a 

 force far inferior to that by which it had formerly been tested. Many suppositions ware 

 made to account for this phenomenon, when finally a person took a series of rods .about the 

 eize of pipe-stems, all strong and tough, and, with great patience, allowed thjm to fall 

 for hours and hours upon an anvil, thus producing rapid strokes and vibrations. After 

 subjecting them for a long time to this treatment, he found tliat the rods could be snap- 

 ped aa L broken iuto fragmcutu almost as easily as rotten wood. 



