for Measuring the Expansion of Solids. 263 



silver, the calculation of the temperature of melting iron, which 

 is more than one third higher, would be sensibly affected by 

 it. I think it therefore extremely probable that the true tem- 

 perature of melting cast-iron is below 2786°. 



The consistency of these results will, I trust, remove any 

 doubts as to the competency of the pyrometer to determine 

 fixed and comparable points of very high temperatures, and 

 induce those connected with arts and manufactures to intro- 

 duce its use, for the purpose of ascertaining many questions Oi 

 the highest interest, both to practical and theoretical science. 

 The experiments just detailed upon bars of wrought-iron re- 

 moved even the only trifling objection which could be brought 

 against its general use; namely, the expense of a platinum 

 bar : for it is quite proved that a bar of wrought-iron is suffi- 

 cient for every practical purpose, and it affords the important 

 additional advantage of a much more open scale. 



I proceed now to remark that zinc, as well as iron, appears 

 by the Tables to present an exception to the law of an in- 

 creasing rate of expansion with increasing temperature ; the 

 expansion for the 600° to boiling mercury not being so much 

 as four times that for the 150° to boiling water. I cannot, 

 however, from some peculiar circumstances attending the ex- 

 periment, place entire confidence in the result. When, after 

 boiling in mercury, the register was opened, the vapour was 

 found to have gained admittance, and to have acted upon the 

 zinc. It was firmly fixed in the cavity, and was not removed 

 without considerable difficulty and piecemeal. At its upper 

 end, the bar was reduced almost to a point, and was very con- 

 siderably thickened at its lower end, and moulded to the 

 bottom of the register, as if it had been partially fused. It was 

 hard and brittle. I'he vapour of the mercury had probably 

 combined with it at some temperature below the boiling point; 

 the amalgam so formed had flowed down to the bottom of the 

 bar, and the mercury was afterwards expelled by the boiling 

 temperature. 



I may here observe, as not unworthy of attention, that in 

 no instance have I seen a metal acted upon by the vapour of 

 mercury at its full boiling temperature ; — even gold, which 

 has so strong an affinity for it, comes out of it with its yellow 

 colour perfectly unstained ; but when the mercury is in the 

 fluid form at the same temperature, the gold is immediately 

 dissolved by it. 



Under these circumstances there certainly may exist some 

 doubt whether the full amount of expansion in zinc to the 

 boiling j)oint of mercury was properly registered. 



On tlie other hand, in confirmation of the result so re- 



