158 Mr. Mallet on the Physical Conditions 



35. The amount of /mea^ contraction due to solidification of cast-iron appears 

 to vary with metal and circumstances of casting, from y^ up to -g-L of the dimen- 

 sions of the cold mass. Its contraction in volume, therefore (more than three 

 times this), and probably not equal in the directions of three rectangular axes, 

 owing to the crystalline structure, is so great, and the difference such, between 

 its measure for large and small parts of the same casting, that the latter never 

 should be neglected. 



36. The effects of this difference are well known to founders by causing 

 castings of certain forms to become distorted or spontaneously broken after they 

 have solidified. To multiply instances would be tedious ; but one circumstance 

 requires remark, as proving that these internal strains occurring in castings of 

 variable bulk exist where little suspected, and that it is with extreme slow- 

 ness that the molecules after consolidation appear gradually to assume minute 

 changes of arrangement, and to adjust themselves, within certain limits, to a 

 state of permanent equilibrium. 



37. It is a fact known well to working mechanics engaged in boring or turn- 

 ing or otherwise cutting into large castings of iron that have cooled safely and 

 without crack or flaw, that yet when a part of the whole mass shall have been 

 cut away — as for example, when a large and thick-flanged cylinder, or a large 

 toothed wheel, or other irregular discoid mass, is " bored out," the form of the 

 exterior of the mass changes during the operation. The portion cut away 

 destroys the temporary equilibrium that was established in the mass, and it 

 again changes its form, and perhaps its symmetry, and sometimes even its 

 volume. 



38. For some most valuable illustrations of the singular forms or lines of 

 direction which the curves of internal tension and compression take in solids of 

 various forms thus under elastic constraint, Mr. Maxwell's paper in the Trans- 

 actions of the Eoyal Society of Edinbiu-gh, vol. xx. part i., may be consulted. 



They bear a remarkable analogy to the nodal lines of Chladni and Savart 

 traced by their researches on the vibrations of sonorous plates, and are directly 

 connected with the optical properties first shown by Sir David Brewster in 

 glass under constraining forces. 



39. Sometimes a casting which has cooled safely will fly to pieces on receiv- 

 ing a sudden jar or blow, of a trifling degree offeree — a fact which is in analogy 



