-MECHANICS AND USEFUL ARTS. 53 



made of iron, or partly of iron and partly of sand, that portion of the 

 casting which has run against the iron becomes what is technically 

 termed '.chilled,' and is indicated by a white crystalline structure to 

 a depth depending upon various conditions of temperature of the mould 

 and the metal run into it, as well as of the chemical composition of the 

 iron. The practical utility of chill-casting depends on the fact that 

 the part thus rendered crystalline is of extreme hardness, nearly equal 

 to that of hardened steel, whilst the remainder of the casting may be 

 as soft as iron cast in the ordinary sand mould. The rationale of the 

 effect thus produced is not well understood. Cast-iron is a compound 

 of iron with variable proportions of carbon, and these proportions have 

 not, as I believe, been yet reduced to anything like atomic order ; some 

 statements give as much as 15 per cent, of carbon in very soft pig-iron, 

 and such iron exhibits very little or no tendency to chilling. Practical 

 experience is at present the only guide to the production of the desired 

 effect ; in some cases a very thin hard stratum is desired, in others a 

 considerable depth ; and this stratum may be varied from an almost im- 

 perceptible white line to half or three quarters of an inch hi depth ; 

 this latter being required in the larger rolls for making the finest thin 

 sheet-iron. Chemically speaking, cast-iron and steel are of the same 

 composition, viz., iron with a proportion of carbon ; the proportion of 

 the latter in cast-iron being infinitely greater than in steel. Here I 

 would point out a remarkable difference between chilled cast-iron and 

 steel. If the latter is heated red-hot, and plunged into cold water, it 

 becomes extremely hard ; if, in this state, it be again heated, it re- 

 sumes its original softness ; but if chilled iron be so treated, it still re- 

 tains its hardness. Whether this is caused by mere mechanical arrange- 

 ment or by the chemical combination of the atoms, whether there be 

 a metallic base of carbon in one case and not in the other, or by what- 

 ever these differences are caused, is far too little understood. The 

 whole subject is one deserving the close attention of those whose pur- 

 suits enable them to study chemical analysis. Indeed, when we reflect 

 on the fact, that, without the peculiar properties of iron and carbon, 

 civilization could not have been carried on, it does appear strange that 

 the master minds of the age have not acquired more knowledge of the 

 relative action and combination of these two substances. It would be 

 foreign to our present object to enter upon the mode of manufacturing 

 steel ; but I may state the fact that it is extremely difficult to procure 

 any masses that are of uniform density, whilst chill cast-iron is easily 

 produced with large homogeneous surfaces : and this brings me to the 

 main subject proposed for your attention, viz., the apph'cation of it to 

 pivots of astronomical instruments. About four years since, the Astron- 

 omer Royal applied to ray partners and self respecting the construction 

 of the mechanical parts of a new meridional instrument, the size of 

 which so greatly exceeded anything of the same kind, that it became a 

 serious question of what material the pivots should be made ; it was 

 requisite that it should be both hard to resist wear as much as possible, 

 and homogeneous to insure that whatever wear took place should be 

 uniform. The extensive use we make of chill cast-iron suggested that 

 if the pivots were so cast with the body of the axis in sand moulds, and 



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