involved in the Construction of Artillery. 297 



intense white heat; wrought-iron is perfectly annealed at a clear bright red 

 (about 1200° Fahr., according to the experiments of the Franklin Institute) ; 

 copper anneals perfectly at a very low red heat, scarcely visible in clear day- 

 light; and zinc at a still lower temperature. Some metals, of very low fusing 

 points, such as lead and tin, probably owe their apparent incapability of 

 becoming hardened by lamination or wiredrawing, to their anneahng tempe- 

 ratures being so low, that the heat evolved in the process, is sufficient to anneal 

 them, i. e. to prevent that change in the mutual relations of the particles, 

 whether one of distance or of position, upon which hardening depends. 



A rich reward awaits the physicist who, in a comprehensive manner, shall 

 first, experimentally, attack the question of the molecular changes produced by 

 hardening and annealing ; it has been as yet almost unattempted. As respects 

 the material with which we are immediately engaged, — wrought-iron, — Baudri- 

 mont, in a very valuable paper ("Ann. de Chim. et Phys.," t. ix.), appears to 

 have ascertained, that a temperature above that of " cherry red," perhaps, about 

 1150° Fahr., is necessary for annealing it ; — that at a white heat it is almost 

 instantly annealed, and at the same time suiTers more or less a change of crys- 

 talline structure ; — and that a certain amount of tenacity is lost by annealing 

 platinum, iron, copper, and some of its alloys. 



298. The changes in volume, or density, induced by mechanical pressure, 

 and by annealing of the same wrought-iron, he found, by some delicately con- 

 ducted experiments, as follows: — 



Specific Gravity. 



Iron wire compressed by the draw-plate, . 7'6305 



„ annealed, 7-6000 



Iron laminated, 77169 



The same, annealed, 7-6000 



„ laminated a second time, .... 7-7312 



„ hammer-hardened, 7-7433 



His results for copper, which probably, judging from Dussausoy's results, 

 would approximately apply to gun-metal, are as follow: — 



Copper, fused and cooled slowly, . . . . 8-4525 

 „ compressed by the draw-plate, . . 8-6225 

 „ annealed, 8-3912 



y 



