320 Dynamic Theory. 



dissipated in the steam which is exhausted into the air after the work is 

 done, in the hot water which never becomes steam, in the heated ma- 

 chineiy, &c. And after a part of it does become work a considerable 

 percentage is degraded back to heat by the friction of the machine^. 

 The best engine utilizes only about ^ of the heat in work, yet no part of 

 the molecular energy is lost. It continues in some form, either molecu- 

 lar or molar, either heat, light, &c. , or work. But an apparent excep- 

 tion to the foregoing is to be observed when work is done against the 

 attraction of gravitation. If a bag of wheat be raised fift} T feet to the 

 top floor of a mill and left there at rest, there is an end of the work done 

 to get it there and no increase of heat or other sort of motion to indicate 

 what has become of the work. But if the bag should drop from the top 

 floor to the bottom, the heat generated by the concussion, together with 

 such other results as might happen in splitting the bag and scattering 

 the grains, &c. , would, in all, exactly equal the work done in getting it 

 to the top in the first place. So that while it is above, it is said to oc- 

 cupy a position of energy. Its energy is potential, but not active. If 

 it be dragged from one end of the upper floor to another, work is con- 

 sumed in the operation and passes into heat in the contents of the bag 

 and the molecules of the floor over which it passes, but the potential 

 energy is neither increased nor diminished. Work done against chemi- 

 cal attraction as well as gravity ends in the same way by leaving the 

 bodies separated by such work in a position of potential energy. The 

 work done by sunlight in the leaves of plants is a compulsory separation 

 of oxygen from carbon, the carbon taken from carbonic acid being 

 packed away in the tissues of the plant, while the oxygen is dismissed 

 into the atmosphere. Thus separated by force the two occupy a potential 

 attitude toward each other. When the carbon in the shape of a stick of 

 wood or a piece of coal is started to vibrate with enough rapidity to 

 loosen the cohesion of the atoms, as may be done by friction or the appli- 

 cation of fire, the reunion of the oxygen with the carbon takes place and 

 the same amount of energy, in the shape of heat, is given off, that was 

 required to disunite the two in the first place. Work done against 

 cohesion, however, as when a log or a rock is split with a maul and 

 wedge, is converted into the molecular energy of heat. It is parallel 

 with the case of dragging the bag of wheat over the floor. The process 

 does not alter the potential relations of the parts, but is only a method 

 of degrading molar into molecular energy. 



The two different modes of motion are thus found to be convertible 

 into each other. But each of these two modes of motion, as before ob- 

 served, is subdivisible into a number of varieties and phases. Every- 

 one is familiar with the mechanical contrivances by which molar energy 

 of one simple kind is caused to take on a great variety of motions. 



