Mechanical Science, 167 



even months later. It frequently happens, that a body which pro- 

 duced with difficulty only dull sounds, finally vibrated with such 

 facility and energy, that its particles became disintegrated, and it 

 flew to pieces upon the slightest agitation of its parts. From hence 

 it appears to result that, during the solidification, many parts are, 

 as it were, surprised in positions, from which they tend to depart, 

 and that they acquire a permanent state of equilibrium only after a 

 long time : thus, if a circular plate of sulphur be cast in a mould, 

 and endeavours be made, immediately after cooling, to produce 

 sound from it, no sound can be obtained. After some days, sounds, 

 more or less dull, may be obtained ; if then the number of vibrations 

 for any set of nodal lines be determined, and then the plate be left 

 for a month or two after that time, it will be found to sound freely, 

 and further, the same set of nodal lines, or mode of division, will 

 give a greater number of vibrations ; the sound may thus be raised 

 sometimes a full tone. It is well known that sulphur, which has 

 been recently fused, does not immediately recover its former proper- 

 ties ; but no one suspected that it required whole months, and 

 even a longer period, to fully restore them. — Annates de Chimie, 

 xli. p. 61. 



9. On the Solidification of Plaster, by M. Gay Lussac. — Every 

 one knows the property which plaster possesses, when deprived of 

 its water by heat, of becoming solid with that fluid. The consist- 

 ency which it acquires is very variable, and the purest plasters are 

 precisely those which acquire least hardness. The cause has been 

 attributed, in Paris plaster, to the presence of a few hundredths of 

 carbonate of lime ; but, without doubt, erroneously; for the heat 

 necessary to bake the plaster is, in the small way, not above 300° R, 

 and, in the large way, is never carried to the degree necessary to 

 decompose the carbonate of lime. Besides, calcined plaster rarely 

 contains free lime, and the addition of that base to those plasters, 

 which have but little consistence, does not sensibly improve them. 

 I think that we must search for the difference of consistency, which 

 is acquired by different plasters, when mixed with water, in the 

 hardness which they possess in then* natural uncalcined state ; a hard- 

 ness which we cannot explain, but must take as a natural fact. 

 That stated, I suppose that a hard plaster-stone, having lost its 

 water, will acquire greater consistency when returning to its first 

 state than a plaster-stone naturally softer. It is in some degree 

 the primitive molecular arrangement which is reproduced. We 

 find, in the same way, that when good fused steel has its carbon 

 removed by cementation with oxide of iron, it will give, by a new 

 cementation with carbon, a steel much more homogeneous and 

 perfect than that obtained in the same circumstances by the ce- 

 mentation of iron. — Annates de Chimie, xl. p. 436. 



10. Formula for reducing a Mercurial Thermometer in high Tem- 

 peratures. — If q denote the degrees of a mercurial thermometer, n 



