Some bodies allow the heat rays to pass through them, as air does, and as rock salt will do. White clothing is preferable in summer (and also in winter if we could only make people believe it). White garments radiate less heat in winter, and absorb less heat in summer. An old black kettle will boil water more quickly than a new bright one, but the latter will keep the water hotter for the longer time when not on the fire. Heat, then, is movement of particles. Energy can be changed into heat, as the savage finds when he rubs the bits of wood to produce heat and fire. Friction causes heat, and chemical combination produces heat; and, if “visible energy can be turned into heat, heat can be turned back into visible energy.” For fire heats water, water expands into steam, and steam produces motion and energy in the steam-engine. If we heat water in Wollaston’s bulb,—the opening of which is hermetically stopped by a piston,—the vapour will raise the piston. If we cool the bulb we condense the steam, and the piston falls. Here we have the principle of the steam-engine. STEAM is the vapour of water educed by heat, and we may give a few particulars concerning it. Its mechanical properties are the same as those of other gases, and pure steam is colourless and transparent—in fact, invisible. Its power when confined in boilers and subjected to pressure is enormous, for the volume of the steam is far greater than the water which gave rise to it. One cubic inch of water will produce 1,700 cubic inches of steam—in other words, a cubic inch of water produces a cubic foot of steam. When we obtain steam at 212°, we do so under the pressure of one atmosphere; but by increasing the pressure we can raise the boiling point, and thus water at the pressures of sixteen atmospheres will not steam till it reaches 398°. It is thus we obtain pressure for locomotives, and other engines, although a very small portion of the steam does work. Much the largest portion is expended in overcoming cohesion, and one way and another, taking into consideration defects in machinery, only about one-tenth of the heat is employed in doing the work. The force exercised by steam under atmospheric pressure is sufficient to raise a ton weight one foot. To obtain very high temperatures we shall find the thermometer of no use, for mercury boils at 662°, so an instrument called a Pyrometer is used to ascertain the fusing point of metals. Mr. Wedgwood, the celebrated china manufacturer, invented an instrument made of small cylinders of clay moulded and backed, placed between two brass rods as gauges divided into inches and tenths. But this instrument has been long superseded by Professor Daniell’s Pyrometer, which consists of a small bar of platina in an earthenware tube. The difference of expansion between the platina and the tube is measured on a scale on which one degree is equal to seven degrees of Fahrenheit. Thus the melting temperatures of metals are ascertained.