EXPANSION, from the Latin expand°, I expand,' is a term employed in physics, to denote that increase of magnitude which almost all bodies, whether solid, fluid, or aeriform, receive from an increase of temperature. The instruments by which the expansions of solid bodies are measured, are named Pyrometers, which were ori ginally constructed by means of multiplying wheels, screws, and levers ; but they have recently been con structed with micrometer microscopes, which enable us to measure the expansion of solids with singular accura cy. By means of a pyrometer, Mr Ellicot obtained the following expansions for seven metallic bodies, by the same elevation of temperature.
Gold. Silver. Brass. Copper. Iron, Steel. Lead. 73 103 95 89 60 56 149 Mr James Ferguson obtained the following measures : Brass. Copper. Iron. Steel. Lead. Tin.
5 4 3 3 7 6.
M. Berthoud obtained the following results with rods three feet two inches and five lines long, five lines wide, and three lines thick, which were placed in a situation where the thermometer rose from 0° to 27° (of Reaumur, probably). The measures are given in parts of a line.
Steel tempered Brass. Copper. Iron. and blued.
121 107 78 77 360 360 360 360Experiments of a similar kind were made by Smeaton, Ramsden, Muschenbroek, Borda, General Roy, and Mr Troughton.
In 1794, Mr Troughton made a pyrometer for Captain .Mendoza, similar to that made by Ramsden, and descri bed by General 'Roy in the Philosophical Transactions for 1785, and the instrument having remained some months in his possession, he embraced the opportunity of examining the expansions of brass, steel, cast iron, and flint-glass. The following were the results which he obtained Brass .000640. Expansion on one inch with 60° of the mercurial thermometer. Steel .0003966. Ditto. ditto.
The results for cast iron and flint glass he has unfor tunately lost. Although these measures were obtained after frequent repetitions of the experiments, made with the greatest care, yet they differed considerably from those obtained by General Roy.
In 1794, Mr Troughton constructed a pyrometer* on a new principle ; hut as it was only designed for trying pendulums in a finished state, a length greater than 40 inches could not be subjected to trial. With this instru ment he obtained the following results : Silver 000694.
Expansion on one inch with . • . .
60° of the mere. therm.
Iron wire • .000480. do. do.
Hard drawn Platina . . .0003305. f do. do.
Mr Troughton considers all the experiments which have hitherto been made on the expansion of solids, as undeserving of confidence, and he proposes to construct a new pyrometer of the kind already mentioned, so as to be able to receive a ten feet rod. A series of experi
ments made with such an instrument, and by a philoso pher of Mr Troughton's known accuracy, will be a most acceptable present to the physical sciences, and will probably lead to a determination of the law which regu lates the expansion of solid bodies.
The results of the best experiments we have given in the following valuable Table; for which, with the excep tion of a few additions, we are indebted to Dr Thomas Young. It exhibits the increase both of length and bulk for 180°, and for I° of Fahrenheit's scale.
It has been found, in general, that equal increments of heat produce a greater expansion at a high temperature, than they do at a low one.
The expansion of metallic bodies seems to have some connection with their fusibilities. Platina, the least fusible of the metals, has the lowest expansion ; and lead, one of the most fusible, has the greatest expansibility. The most fusible glass is also found to be the most expansible.
From a number of experiments made with Prince Rupert's drops, which are formed by dropping melted glass into cold water, Dr Brewster has endeavoured to obtain a measure of the expansion of glass when in a fluid state. When the drops are made of flint glass, they contain a number of cavities formed by the con traction of the glass in cooling. These cavities contain no air. They increase with the magnitude of the drops, and may therefore be considered as a measure of the contraction which the glass undergoes in its transition from the temperature at which it melts, to the ordinary temperature of the atmosphere. The magnitude of this contraction is measured by the difference between the specific gravity of the annealed and the unannealed glass. This measure of the contraction must always err in defect, but the maximum result obtained from a considerable number of drops, may be regarded as a tolerably correct measure of the diminution of density. Those drops should be employed in which the cavities are most numerous, and scattered over every part of their length. In some large drops of flint glass, the number of cavities is given. Dr Brewster found that the specific gravity of unannealed flint glass was 3.20405, and that of annealed flint glass, from the same pot, 3.2763. Hence the expansion in passing from the fluid to the 1 solid state, is greater than — , or 0.002205. Sec the 45.3 Phil. Trans. for 1315, p. 1. where a full account of these experiments is published.