Mr. Ivory on the new Tables of Refraction. 261 



ferent elasticities of the vapour. The longitudinal expansion 

 of the hot part of the tube or of its contents cannot sensibly 

 affect the results; but as the tube widens, the column of water 

 will shorten, and consequently its hydrostatic pressure on the 

 mercury will be diminished in a very small degree. It is cu- 

 rious to observe what strange mistakes regarding the effects 

 of the expansion of the tube, one of our first-rate authors has 

 fallen into, while playing the critic on Dr.Ure s experiments. 



In making experiments on aqueous vapour at a lower tem- 

 perature than that of the apartment, the whole column of 

 water or mercury should be reduced to the temperature of the 

 vapour; otherwise, on account of the facility with which cold 

 descends in fluids, the result might be uncertain But if the 

 water be frozen, its adherence to the tube wi 1 obstruct the free 

 motion of the mercury. It must therefore be very difficult to 

 make accurate experiments on aqueous vapour at or below the 

 freezing point: because the vapour may be pajMly con- 

 densed if in contact with glass, or the motion of the fluid* 

 may be liable to obstruction in the tube. Besides, n these 

 cases the mercurial column being about 30 inches long, an 

 error' in \T temperature may materially affect the observed 

 elasticity, which is then a very small quantity. 



Yours 8cc. 



TJ 



March 29, 1824. 



XLIII. Remarks on an Article published in No .23 of the 

 Journal of Science, and treating ff*™ ™ es <* Re ~ 

 fraction. " By J. Ivory, Esq. M.A. F.R.S. 

 To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 

 IF vou can possibly find room, I trust you wil insert, m 

 I vou, next publication, the following observations occa- 

 sio,Sl b V an aAicle on my Table of Refractions that appeared 



iU W^pa^uWbS" the Philosophical Transacts 

 or unexplained «5>^ft^jSha elementary quantities 



