54 NATURAL HISTORY. 



86. Another experience there is of induration by 

 cold, which is already found ; which is, that metals 

 themselves are hardened by often heating and 

 quenching in cold water : for cold ever worketh 

 most potently upon heat precedent. 



87. For induration by heat, it must be consi 

 dered, that heat, by the exhaling of the moister 

 parts, doth either harden the body, as in bricks, tiles, 

 &c. or if the heat be more fierce, maketh the grosser 

 part itself run and melt ; as in the making of ordi 

 nary glass ; and in the vitrification of earth, as we 

 see in the inner parts of furnaces, and in the vitrifi 

 cation of brick, and of metals. And in the former 

 of these, which is the hardening by baking without 

 melting, the heat hath these degrees ; first, it in- 

 durateth, and then maketh fragile ; and lastly it 

 doth incinerate and calcinate. 



88. But if you desire to make an induration 

 with toughness, and less fragility, a middle way 

 would be taken ; which is that which Aristotle hath 

 well noted ; but would be thoroughly verified. It 

 is to decoct bodies in water for two or three days ; 

 but they must be such bodies into which the wate 

 will not enter ; as stone and metal ; for if they be 

 bodies into which the water will enter, then long 

 seething will rather soften than indurate them ; as 

 hath been tried in eggs, &c. therefore softer bodies 

 must be put into bottles, and the bottles hung into 

 water seething with the mouths open above the water, 

 that no water may get in ; for by this means the vir 

 tual heat of the water will enter ; and such a heat, as 



