Expansion, in Natural Philosophy, the enlargement or increase of bulk in bodies, chiefly by means of heat. This is one of the most general effects of caloric, being common to all bodies whatever, whether solid, fluid, or in the aeriform state. Metals expand in the following order, those that expand most being placed first: - zinc, lead, tin, copper, bismuth, iron, platina. The degree of expansion produced in different liquids varies considerably. In general, the denser the fluid, the less the expansion; water expanding move than mercury, and alcohol more than water. The various elastic fluids, or gases, on the contrary, all expand equally, the expansion being about one four hundred and fortieth part of their bulk at 32° Fahr, for every degree of heat. But elastic fluids are capable of expanding indefinitely without the application of heat, by the mere enlargement of the containing vessel; since whatever be its capacity, they must necessarily be equally diffused, and press with equal force in every part of it, the pressure being inversely as the bulk of the gas.
This property of elastic fluids has been turned to great advantage in steam engines, by admitting steam of high pressure into the cylinder during a portion of the stroke, and then shutting off the communication with the boiler; the expansion of the steam in the cylinder carries the piston with a constantly decreasing force through the remaining portion of the stroke, by which mode of working the whole effect produced during the expansion of the steam is clear gain. Upon this subject Mr. J. Perkins, who has employed steam of greater expansive force than perhaps any other person, observes, that there is great economy in using very high steam expansively, and that the higher the steam can practically be used, the sooner it may be cut off. The preceding diagram shows (approximately) the gain in cutting off the steam at a quarter stroke. Let the piston, which is represented by the line k\ a descend to i b, being one quarter of the stroke, with a constant force of 400 lbs. per square inch. At this point let the steam be cut off and expand to double its volume; when it arrives at h c it will be exerting a pressure of 200 lbs. per square inch, producing a mean pressure of 300 lbs. per square inch through the quarter stroke.
Let the steam again expand to double its volume, and the piston will finish its stroke at f e at 100 lbs. per inch, giving a mean of 150 lbs. through the last two quarters; to which add 400=the pressure during the first quarter, and 300=the pressure during the second, and the sum will be 1,000, giving a mean pressure of 250 lbs. on the inch throughout the whole stroke. It will be seen that when the stroke is completed, the cylinder will be filled with steam of 100 lbs. pressure per square inch, which will be the same in quantity as though the steam had begun at a pressure of 100 lbs. and continue at that pressure throughout the stroke; but in this case the sum of the pressure through the four quarters would only be 400 lbs. so that by using the same quantity of steam expansively, there is a gain of 150 per cent. By employing two cylinders, the pistons of which act upon cranks at right angles to each other, a compensation is obtained for the varying pressure of the steam; for whilst one piston is at its greatest power, the other is acting with diminished power, so as to render the force exerted nearly the same throughout the revolution, as will be seen by the diagram. The annexed figure represents an instrument for showing the expansive force of steam at different temperatures.
At the bottom of a strong spherical vessel of brass is placed a quantity of mercury sufficient to fill the long vertical glass tube above; over the mercury is the water to be converted into steam, by a spirit lamp placed beneath. The long tube is submerged in the fluid, so as to nearly touch the bottom; on one side of this tube a thermometer is fixed in an inclined position, its bulk projecting towards the centre of the vessel. On the application of heat, the water is converted into steam, which, by its expansive force, presses upon the surface of the mercury, and impels it up the long tube, where its pressure is noted upon a graduated scale; at the same time, the height of the mercury in the thermometer shows the temperature of the steam.