ENERGY. 435 



seen heat changed to electricity and vice versa, and either 

 or both changed to mechanical energy. It does not mean 

 that the sum of these eight variable quantities in the earth 

 is Constant, for we have seen that energy may pass from 

 sun to earth, from star to star. But it does mean that the 

 sum of all these energies in -all the worlds that constitute 

 the universe is a quantity fixed, invariable. 



676. Correlation of Energy. The expression 

 Correlation of Energy refers to the convertibility of one 

 form of energy into another. Our ideas ought, by this 

 time, to be clear in regard to this convertibility. One im- 

 portant feature remains to be noticed. Kadiant energy can 

 be converted into other forms, or other forms into radiant 

 energy only through the intermediate state of absorbed 

 heat. 



677. A Prose Poem. "A river, in descending from an 

 elevation of 7720 feet, generates an amount of heat competent to 

 augment its own temperature 10 F., and this amount of heat was 

 abstracted from the sun, in order to lift the matter of the river to 

 the elevation from which it falls. As long as the river continues 

 on the heights, whether in the solid form as a glacier, or in the 

 liquid form as a lake, the heat expended by the sun in lifting it 

 has disappeared from the universe. It has been consumed in the 

 act of lifting. But, at the moment that the river starts upon its 

 downward course, and encounters the resistance of its bed, the heat 

 expended in its elevation begins to be restored. The mental eye, 

 indeed, can follow the emission from its source through the ether, 

 as vibratory motion, to the ocean, where it ceases to be vibration, 

 and takes the potential form among the molecules of aqueous vapor ; 

 to the mountain-top, where the heat absorbed in vaporization is given 

 out in condensation, while that expended by the sun in lifting the 

 water to its present elevation is still unrestored. This we find paid 

 back to the last unit by the friction along the river's bed ; at the 

 bottom of the cascade, where the plunge of the torrent is suddenly 

 arrested ; in the warmth of the machinery turned by the river ; in 

 the spark from the millstone ; beneath the crusher of the miner : in 



