412 



THE POPULAR EDUCATOR. 



Fig. 17 



platinum wire, it will render it white-hot, and a considerable 

 amount of light will be produced, showing again the luminous 

 effects of heat. We must not, however, suppose that heat is 

 always accompanied by light, or light by heat. The electric 

 lamp furnishes us with a very brilliant light and at the same 

 time an intense heat, so that we have both the luminous and 

 the calorific rays in a beam from it. If now we cause this beam 

 to pass through a glass trough filled with a solution of alum, 

 the luminous rays will pass on as before, but all or nearly all 

 of the heat will be intercepted. The alum solution serves, in 

 fact, as a filter to remove the thermal rays. Now remove the 

 glass trough, and 'substitute for it a slab of rock-salt thickly 

 covered with lamp-black, so that no light can penetrate it. On 

 placing a differential thermometer, or thermo-electric pile, in 

 the place where the luminous 

 rays had previously been brought 

 to a focus, we shall find that 

 nearly all the heat has passed 

 through the rock- salt, though the 

 luminous rays have been inter- 

 cepted. By suitable arrange- 

 ments we may actually succeed 

 in igniting various substances 

 by means of this non-luminous 

 heat. We see thus that the 

 luminous and the heat-giving 

 rays may be entirely separated 

 from one another. 



We have now to notice the 

 mechanical effects of heat, and 

 to learn how it may be con- 

 verted into work. To ascertain 

 the mechanical equivalent of heat 

 that is, the amount of work 

 that can be accomplished by a 

 given quantity of heat is a diffi- 

 cult proposition. It has, how- 

 ever, been solved, mainly by the 

 patient researches of Drs. Joule 

 and Meyer. The following expe- 

 riments will give an idea of the 

 process adopted by the latter : 



Let A B (Fig. 17) be a tube 

 closed at its lower end, having a 

 sectional area of one square inch ; 

 and let c be a piston fitting it 

 air-tight, and capable of moving 

 up and down without friction. 

 Also let c be supposed to weigh 

 15 Ibs. 12 oz., and to be 492 

 inches from the bottom, the 

 air below being at the freezing 

 point. Now raise the tempe- 

 rature of the air 1, and since 

 the co-efficient of expansion is 

 -tfo, the piston will rise one inch, 



and be 493 inches from the bottom ; and thus, for every degree 

 the temperature is raised, the piston will rise an additional 

 inch. If then, the temperature is raised 492, the volume of 

 air will be doubled. In this case work has been done by the 

 heat, and that work has consisted in raising the piston and the 

 air above it, which together press with a force of 15 Ibs. + 15 Ibs. 

 12 oz., or 492 oz., to a height of 492 inches. 



Now try the experiment in a different way, and ascertain the 

 additional weight requisite to keep the piston in its place, while 

 the temperature varies. We shall find that if the temperature 

 is raised 1, one ounce must be added to the piston to keep it 

 stationary ; if 2, two ounces, and so on. Hence, if the tem- 

 perature be raised 492, 492 oz. must be placed on the piston 

 to keep the volume the same. Compare now these two experi- 

 ments. In one case we have raised the temperature, keeping 

 the pressure constant while the volume increased ; in the other 



15. 



case, the volume has been kept constant. The same amount of 

 air has been raised in each case to the same temperature ; but 

 a different qiiantity of heat has been required ; for investigation 

 shows that if 10 grains of any combustible material are required 

 when the volume is kept constant, 14'21 grains of the same 

 material would be required when the pressure remains unaltered. 

 The extra 4'21 grains, then, has been employed in raising the 

 weight, and has thus been converted into work. 



Now suppose we have a vessel of air one square foot in area, 

 and raise it 492 in temperature, the air will occupy double the 

 space; and as the pressure on its surface is 144 x 15 Ibs. = 2,160 

 Ibs., it will have lifted this weight one foot, or, in other words, 

 performed work amounting to 2,160 foot-pounds. The weight 

 of the cubic foot of air is T29 oz., and, as will be explained 



shortly, the amount of heat re- 

 quired to raise this to any tem- 

 perature would only raise 0'31 

 oz. of water to the same tempe- 

 rature, the air having less capa- 

 city than the water. The total 

 amount of heat, then, which 

 has been received by the air is 

 sufficient to raise 0'31 oz. of 

 water 492, which is the same 

 as raising 9* Ibs. 1. Of this 

 amount, ^ T is, as explained 

 above, employed in driving back 

 the air, while the rest serves to 

 raise the temperature. Now 

 2 5 1 T of 9 Ibs. is about 2'8 Ibs., 

 and thus we find that the 

 amount of heat required to raise 

 2 '8 Ibs. of water 1 is sufficient 

 to elevate 2,160 Ibs. to a height 

 of 1 foot. Dividing 2,160 by 

 2'8, we get a quotient of 772 

 nearly, that is, the quantity of 

 heat required to raise a pound 

 of water 1 will perform work 

 equivalent to 772 foot-pounds. 

 As, however, the thermal unit 

 is usually taken as 'the quan- 

 tity required to raise a pound 

 of water 1 in the Centigrade 

 scale, the equivalent must be 

 increased by |, and will be found 

 to be 1,390 foot-pounds. 



By a number of various 

 experiments, conducted with 

 great care and patience, Dr. 

 Joule arrived at a very simi- 

 lar result, and we may there- 

 fore safely take this as the 

 true equivalent. The amount 

 seems very large, especially 

 when we consider the great 



amount of heat produced by the combustion of various sub- 

 stances. A pound of charcoal, for instance, by its combus- 

 tion produces 8,000 units of heat, and thus generates a force 

 sufficient to raise a weight of nearly 5,000 tons to a height of 

 one foot. 



We do not wonder, since this is the case, that means should 

 have been sought of utilising the heat of the sun's rays, 

 which, on a bright summer day, are calculated to impart 

 about 5 thermal units per minute to each square foot of 

 surface, placed so as to receive them perpendicularly. No 

 important practical results have, however, been obtained at 

 present from these attempts, though several inventors have 

 claimed for their machines the power of turning this force 

 to good account. It is, however, scarcely probable that, in 

 an economical point of view, they would be able to compete 

 with coal and other articles of fuel. 



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