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IV. Of the Expansion or Contraction os cer-tain Bodies at the Time of their passingfrom a fluid to a solid State.

H E expansion of bodies by heat, and their con-

traction by cold, supposed to proceed always uni-

formly by equal augmentations or diminutionsof heat, appear to have sundry irregularities ; which maydeserve to be taken notice of, not only in a philosophicalview, but likewise as being productive of some effectsinteresting to the workmen.

It has been frequently observed, that when thermo-meters prepared with different fluids, as quicksilver, spiritof wine, water, and oil, have two distant points of heatmarked equally on them all, and the spaces between divi-ded into an equal number of parts ; the heat, whichmakes the fluid in one expand to any of these intermedi-ate points, shall raise that in another above the corre-sponding division, and in another not so high. It was pro-bably this irregularity in the expansion of the fluids, thatprevented the agreement of the mercurial and spirit ther-mometers which Boerhaave fays he had made for him byFahrenheit: the different expansions of different kindsof glass, to which the ingenious artist has recourse inorder to account for the variation, appears to be insuffi-cient for producing it ; since, if the expansion of thetwo tubes be always uniform, or in the same proportionsto one another, the quantity of this expansion cannotinfluence the apparent proportional expansions of thefluids. I have seen a mercurial and spirit thermome-ter very nearly correspond, at different divisions, fromthe freezing point to the heat of melted wax : the di-

visions