Dr. Wollaston, in order to fin-in some estimate of the com parative rate of expansion of piatina and palladium, says, " I riveted together two thin plate: of plat ina and palladium, and observing that the compound plate, when heated, beeame concave on the side of the platina; I ascertained that the expansion of palladium is in some degree the greater of the two. By a similar mode of comparison I found that palla dium expands considerably less than steel by heat." Phil. Trans. jb• 1805.
It must be remarked, with respect to the expansion of glass, that sometimes glass tubes are extended more than solid glass rods; their dilatation, however, is not constant ; for tubes of different diameters, or of diGrent sorts of glass, are expanded differently by the application of like degrees of heat.
Wood is not much expanded longitudinally, that is. in the direction of its fibres, by heat ; and this is particularly the case with deal and other straight-grained wood. Probably, upon the Whole, the longitudinal expansion of wood is less than that of glass. it has been observed. (especially by Dr. ffittenhouse. Torus. of the Phil. Society.) that very dry and seasoned wood, if not exposed to a Very high or to a very low temperature, will expand in length pretty regularly : otherwise its expansion by heat, and its contrac tion by cold, are very irregular ; for they seem to depend partly upon the heat, and partly upon the moisture. which the wood acquires in certain circumstances. and is deprived of in others. It is hardly necessary to mention, that the solids of the preceding table contract their dimensions by cooling, as much as they arc expanded by heating; thus, for instance, if' a yard's length of :My p•tieldar Illetallb" body, by being heated 100 degrees above the act nal temperature of the atmosphere, be lengthened one-fiftieth part of an inch; afterwards, when cooled down to the temperature of the atmosphere, it will be found to have lost exactly that fiftieth part of an inch which it had acquired by heating.
From the experiments hitherto made On the expansions of solids by heat, no correspondence has been observed be tween the expansions and the quantities of calorie they are capable of absorbing. The fusibility of metals seems to coincide with the dilatations; platina, the least ffisible of the metals, dilates the least ; lead dilates most ; ainl the most fusible glass is also the most dilatable. We may therefore conclude with M. Berthollet, that bodies are so much the more expansible, the less calorie they require, to change their constitution front solid to liquid, and from liquid to gases or vapours.
There is a substance which expands xt hen heated, but does not contract %viten cooled ; and of this singular property 3.1r. Wedgwood availed himself for the construction of his in genious thermometer for measuring the highest degrees of heat ; viz., those degrees which exceed the scale of the mercurial thermometer. The substance alluded to is the argillaceous earth or clay, and it appears that the above: mentioned property belongs, more or less, to argillaceous bodies of every kind. This property may at first sight ap pear to be an unaccountable exception from the general law: the difficulty, however, will vanish, if it be considered that bodies of the argillaceous genus contain a considerable quan tity of water, and that the contraction of these bodies, when exposed to the action of a strong fire, is in a great measure due to the escape of the water. and hence they 1 lo not con tract by subsequent cooling.