154 Mr. Mallet on the Physical Conditions 



Some most important conditions, modifying if not invalidating such a con- 

 clusion, and more especially the efiFects of the variable mass of the casting, seem, 

 however, wholly to have escaped him. Indeed, these experiments, rightly con- 

 sidered, only prove what was well known before — that by continually re-melting 

 and casting into small pieces (i. e., imperfectly chilling) any cast-iron, we may 

 gradually cause all its suspended carbon (in the state of graphite) to exude, as 

 Karsten long ago proved, and so gradually convert the metal into an imperfect 

 steel, with increased hardness and cohesion, and diminished fusibility, but with 

 properties altogether unworkable and useless. No such result can occur when 

 the metal is cast into large masses, nor any such assumed improvement by 

 repeated meltings, but very much the contrary. (Note G.) 



26. Again, by some iron-founders, one " make" or sort of pig-iron is presumed 

 to give a closer grain than another, and he prefers it ; and although this 

 is to a certain extent true, i. e., that some cast-irons, that is, some of the innu- 

 merable alloys that go under that name, do iinder equal conditions produce 

 rather smaller crystals than others, still this view equally fails to attain the ob- 

 ject of close-grained, heavy castings. But furthermore, it is a fact familiar to 

 iron-founders, that of several castings of the same form and mass, made at 

 nearly the same time, from the same mixture of metal, and melted in the same 

 furnace, some will, when cold, have a much more coarsely crystalline grain de- 

 veloped in them than others. The fact is familiar ; but I am not aware that 

 any attempt has been made on principle to explain it, and hence no means have 

 yet been prescribed to prevent its occurrence. 



27. Now while the regularity of development of the crystals in cast-iron de- 

 pends, as we have already seen, upon the regularity with which the melted mass 

 cools, and the wave of heat is transmitted from its interior to its surface, arrang- 

 ingthe crystals in the lines of least pressure in its transit, — the extent of development, 

 or, what is the same thing, the size of each individual crystal, depends upon the 

 length of time during which the process of crystalline arrangement is going on, 

 that is to say, upon the length of time that the casting takes to cool. Hence, then, 

 may be announced as a law, that — 



28. The size of crystals or coarseness of grain in castings of iron 

 depends for any given " make" of iron, and given mass of casting 

 upon — 



