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The expansion by heat in different lorms of matter, is exceedingly various. By being heated from 32 to 212 , 1000 cubic inches of iron become 1004 1000 " water " 1045 1000 " air " 1366 Gases are, therefore, more expansable by heat than matter in the other two conditions ot liquid and solid. The reason is, that the particles of air or gas, far from being under the influence of cohesive attraction, like solids or liquids, are actuated by a powerful repulsion for each other. The addition of heat mightily enhances this repulsive tendency, and causes great dilatation. The rate of the expansion of air and gases from increase of temperature, was long involved in considerable uncertainty. This arose from the neglect of the early experimenters to dry the air or gas upon which they operated. The presence of a little water by rising in the state of steam into the gas, on the application of heat, occasioned great and irregular expansions. But in 1801, the law of the dilitation of gases was discovered by M. Gay-Lussac, of Paris, and by Dr. Dalton, of England, independently of each other. It was discovered by these philosophers, that all gases experience the same increase in volume by the application of the same degree of heat, and that the rate ot expansion continues uniiorm at all temperatures. Dr. Dalton confined a small portion of dry air over mercury in a graduated tube. He then placed the whole in circumstances where it was unilormly heated up to a certain temperature, and observed the expansion. Gay-Lussac's apparatus was more complicated bub calculated to give very precise results, He found that 1,000 volumes of air on being heated from 32 to 212 , became 1375, which agreed very closely with Dalton's result.— The expansion was lately corrected by Rud-berg, who found that 1,000 volumes of air expand to 1365. The still more recent and exact researches of Magnus and of Regnault, give as the expansion of air from 32? to 212 d"g, 386-1000, or 11-30 of its volume at 32 deg. The dilatation for every degree of Fah. is 0 002036 (Regnault) ; ,or 1-49T2 part. It follows, consequently, that air at the freezing point expands 1-491 part of its bulk for every added degree of heat on Fah.'s scale : that is— 491 cubic inches of air at 32 deg. become 492 " " S3 aeg. 493 " " 34 deg., c. increasing one cubic inch for every degree A contraction of one cubic inch occurs for every degree below 32 degrees. 491 cubic inches ot air at 32 deg. become 490 " "31 deg. 489 " " 30 deg. 488 " " 29 deg., c. We can easily deduce, from this law, the expansion which a.certain volume of gas at a given temperature will undergo, by heating it up to any particular temperature ; or the contraction that will result from cooling. Air at the temperature of freezing water, has its volume doubled when heated 491 degrees, and when heated 982 degrees, or twice as intensely, its volume is tripled, which is the effect of a low red heat. A slight deviation from exact uniformity in the expansion of different gases was established by the rigorous experiments of both Magnus (Ann. de Chim. c. 3 ser. t. 4, p. 330. et t. 6, p. 363) and Regnault (ibid. t. 4, p. 5 ; et t. 6, p. 370). The more easily liquefied gases, which exhibit a sensible departure from the law ot Mariotte, are more expansible by heat than air, as will appear by the following table :— Expansion upon 1 volume from 32 to 212 degs. Regnault. Magnus. Atmospheric air 036650 0-366508 Hydrogen 0 36678 0 365659 Carbonic acid 0 36866 0 269087 Sulphurous acid 036696 0 385618 .Nitrogen 0-36B82 Nitrous oxide 0 36763 Carbonic oxide 0 36667 Cyanogen - 0-36821 Hydrochloric acid 0 36812 The expansion is also found to be sensibly greater when the gas is in a compressed than when in a rare state; and the results above strictly apply only to the gases under the atmospheric pressure.