On the Coefficients of Expansion, Sfc. 



543 



End section of the trough, showing one of the levers, and the bar which 

 carries it. 



Iodide of Lead. — Pure iodide of lead was cast into rods one-third of 

 an inch in diameter and 6 inches long. The ends were made plane by 

 a fine steel saw, and they were furnished with copper caps. Great 

 difficulty was experienced in casting the rods, owing to the brittleness 

 of the iodide. Slightly greased tubes of very thin German glass were 

 used as the moulds, and as the rods would rarely slip out of the tubes 

 the glass had usually to be chipped away along the whole length of 

 the rod by the point of a knife. The iodide underwent the same 

 changes of colour as were observed in the iodide of silver ; that is to 

 say, it fused to a bromine-red liquid, which, when solidified, became 

 red-brown, and, while cooling, brick-red, reddish-yellow, and, when 

 completely cool, orange-yellow. Harsh noises, like those produced by 

 bending tin, were heard during the cooling of the mass, and the frac- 

 ture was highly crystalline. 



Differences of opinion appear to exist as to the effect of fusing 

 iodide of lead in the air. In the same volume of a standard work I 

 find two exactly contrary opinions : for it is stated, on the one hand, 

 that the iodide if fused in contact with air gives off a part of its 

 iodine, becoming oxyiodide of lead ; while elsewhere the iodide is 

 classed among those which may be fused in an open vessel without 

 change. 



In order to set this matter at rest, 56*1690 grms. of iodide were 

 fused in a covered porcelain crucible. The fusion was continued 



