STEAM. 9 r 



second, and indeed, radiant heat is identical with light. Heat is reflected 

 as is light, and is refracted in the same way as sound. 



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 2 1 2, 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 atmo- 

 spheric 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. 



