involved in the Construction of Artillery. 147 



though often more or less diagonal to the axis of the gun, also mainly follow 

 round, with remarkable regularity, the small re-entering angles made by the 

 several breech or reinforce mouldings on the exterior of the gun ; while those 

 which approach the trunnions, usually fall into the re-entering angles made by 

 these with the body of the piece. 



No doubt these facts are familiar to every " proof master ;" but I am not 

 aware that the value of their correct observation has hitherto been rccoiTuised, or 

 that any attempt has been made to assign a cause for them, or, in fact, to attribute 

 the directions in which a burst gun breaks, to anything more than " accident." 



3. — Causes. Molecular Constitution of Crystalline Bodies. 



9. I proceed to explain the cause. It is a law (though one which I do not 

 find noticed by writers on physics) of the molecular aggregation of crystalline 

 solids, that ivhen their particles consolidate under the influmce of heat in motion, 

 their crystals arrange and group themselves with their principal axes, in lines fer- 

 pen&icular to the cooling or heating surfaces of the solid; that is, in the lines of 

 direction of the heat wave in motion, which is the direction of least pressure within 

 the mass; and this is true whether in the case of heat passing from a previously 

 fused solid in the act of cooling and crystallizing on consolidation, or of a solid 

 not having a crystalline structure, but capable of assuming one upon its tempe- 

 rature being sufficiently raised, by heat applied to its external surfaces, and so 

 passing into it. 



10. For example, — if an ingot of sulphur, antimony, bismuth, zinc, hard 

 white cast-iron, or other crystallizable metal or atomic alloy ; or even any binary 

 or other compound salt or haloid body, as sulphuret of antimony, calomel, 

 sal ammoniac, various salts of barytes and lime, chloride of silver or of lead, 

 chromate of lead ; or even certain organic compounds, such as, camphor, and 

 spermaceti, — provided only it be capable of aggregating in a crystalline form 

 under the influence of change of temperature, as from fusion or sublimation ; — 

 if an ingot or mass of any such body be broken when cold, the principal axes 

 of the crystals will always be found arranged in lines perpendicular to the bounding 

 planes of the mass; that is to say, in the lines of direction in which the wave of heat 

 has passed outwards from the mass in the act of consolidation. 



