26 PHYSICAL FORCE 



now put much, if any, weight on the various origins 

 of force, or, if you will allow me henceforth to use the 

 proper word, the various origins of energy. The 

 energy is the important thing, whether it is brute 

 energy or not the power of working and battling 

 against resistance, either of a living animal or of a 

 mass of dead matter in motion. This theory of 

 energy, or doctrine of work work in the strictly 

 physical sense, not, for example, brain work or 

 artistic work is of vital importance in fields very 

 remote from science. A living being is distinguished 

 from a dead one because it is working every second 

 of its life, and death is the stoppage of that work. 

 But it is not only living things that work continu- 

 ously. A running river, a waterfall, is doing the 

 same. When we speak of this as a live world in 

 distinction to the moon, which is often spoken of as 

 a dead one, we mean not only that there is no life on 

 the moon, but also no movement of anything, and no 

 change of any kind. 



Energy, in general, is due to motion. If the 

 things moving are masses large enough to see, we 

 speak of their mechanical energy. If the things 

 moving are too small to see, even with the micro- 

 scope the molecules or smallest particles of matter 

 that exist we speak of their energy as heat energy. 

 If the particles are still smaller, not matter at all, but 

 electrons or particles of electricity, we speak of their 

 energy as electrical energy. But everything that 

 moves, or has in it the potentiality of movement, 

 possesses energy, and if we trace this energy to its 

 source we find that, in almost every case, it comes 

 from the sun. Trains and ships bear their burdens 

 across land and sea, living creatures run or swim or 

 fly by virtue of energy that comes to us from the sun 

 in the form of radiation, that is, light and heat. In 

 the processes of agriculture this radiant energy is 



