EXCEPTION'S TO THE EXPANSION BY HEAT.

77

SECT. V.EXCEPTIONS

TO

THE EXPANSION BY HEAT.

PHILOSOPHERS have noticed a few exceptions to the law ofheat expanding bodies. For instance; water when cooled downto the freezing point, instead of contracting on the farther depri-vation of heat, actually expands.

Another seeming exception is manifested in alumine or clay ;others occur in the case of cast-iron, and various other metals.Alumine contracts on being heated, and cast-iron, bismuth, &c.when fully fused are more dense than when solid, for as soon asthey become so, they decrease in density, they expand in the actof cooling, and hence the sharpness of figures upon iron whichhas been cast in moulds, compared to that of other metals.

Some philosophers have persuaded themselves that these ex-ceptions are only apparent, but not really true. They say whenwater freezes, it assumes a crystalline form, the crystals crosseach other and cause numerous vacuities, and thus the ice oc-cupies more space. The same is the case with fused iron, bis-muth, and zinc. The contraction of clay is owing to the loss ofwater, of which it loses a part at every increased degree of tem-perature hitherto tried; there is therefore a loss of matter; anda reduction of volume must follow.

Mr. Tilloch* has published a brief examination of the receiveddoctrines respecting heat and caloric, in which these truths aremore fully considered, together with many other interestingfacts relative to the received notions of heat.

Philosophical Magazine, May 18 00, p. 7 i