3 6o NINETEENTH CENTURY. FT. in, 



described in Professor Tyndall's work on * Heat,' where you 

 may also find the great additions that he has himself" made 

 to the work of these men. 



We must content ourselves here with remembering that 

 the physicists of the nineteenth century have shown that 

 heat is *a mode of motion,' and have traced it through 

 all its many wanderings both in earth, air, and sky. They 

 have even followed it from the sun down to our earth, 

 through the plant-world into the beds of coal which are 

 stored up in our rocks, and back again, when this coal is 

 burnt, to the motion which carries our steam-engines and 

 steam-ships across the world. The great German physicist 

 Helmholtz, to whom we owe the greatest modern essay on 

 the conservation of energy, states that *a pound of the 

 purest coal gives, when burnt, sufficient heat to raise the 

 temperature of 8086 pounds of water i Cent. ; ' and from 

 this he calculates 'that the chemical force of attraction 

 between the particles of coal and the quantity of oxygen that 

 corresponds to it is capable of lifting a weight of 100 pounds 

 to the height of twenty miles.' This chemical force was stored 

 up in the vegetables of the coal when they obtained it from 

 the sun's heat ages and ages ago, and now man sets it free 

 to perform the work. 'We cannot create mechanical force,' 

 writes Helmholtz, 'but we may help ourselves from the 

 storehouse of nature. The brook, the wind, which drive 

 our mills, the forest, the coal-bed, which supply our steam- 

 engines and warm our rooms, are the bearers to us of the 

 small portion of the great natural supply which we draw 

 upon for our purposes.' 



Dissipation of Energy Sir William Thomson. But 

 the question still remains whether this natural supply, will 

 always be in such a condition as to perform actual work ; 

 and upon this point our great mathematician, Sir William 



