central parts of the earth to consist of a fluid mass, and that part of this fluid is iron, which requiring a greater degree of heat to bring it into fusion than glass or other metals, remains a solid, and the vis inertiae of this fluid mass with the iron in it, occasions it to perform fewer revolutions than the crust of solid earth over it, and thus it is gradually left behind, and the place where the floating iron resides is pointed to by the direct or retrograde motions of the magnetic needle. This seems to have been nearly the opinion of Dr. Halley and Mr. Euler. NOTE VII.--ELEMENTARY HEAT. _Or sphere on sphere in widening waves expand, And glad with genial warmth the incumbent land._ CANTO I. l. 143. A certain quantity of heat seems to be combined with all bodies besides the sensible quantity which gravitates like the electric fluid amongst them. This combined heat or latent heat of Dr. Black, when set at liberty by fermentation, inflammation, crystallization, freezing, or other chemical attractions producing new _combinations_, passes as a fluid element into the surrounding bodies. And by thawing, diffusion of neutral salts in water, melting, and other chemical _solutions_, a portion of heat is attracted from the bodies in vicinity and enters into or becomes combined with the new solutions. Hence a _combination_ of metals with acids, of essential oils and acids, of alcohol and water, of acids and water, give out heat; whilst a _solution_ of snow in water or in acids, and of neutral salts in water, attract heat from the surrounding bodies. So the acid of nitre mixed with oil of cloves unites with it and produces a most violent flame; the same acid of nitre poured on snow instantly dissolves it and produces the greatest degree of cold yet known, by which at Petersburgh quicksilver was first frozen in 1760. Water may be cooled below 32 deg. without being frozen, if it be placed on a solid floor and secured from agitation, but when thus cooled below the freezing point the least agitation turns part of it suddenly into ice, and when this sudden freezing takes place a thermometer placed in it instantly rises as some heat is given out in the act of congelation, and the ice is thus left with the same _sensible_ degree of cold as the water had possessed before it was agitated, but is nevertheless now combined with less _latent_ heat.