6 LAWS OF ENERGY 



and is thus lost, as far as work is concerned. The quantity of 

 .energy is not decreased, but its dispersion is so great that in 

 quality it has not sufficient potential gradient to be of use. As 

 an example of dispersion of energy, consider a stone dropped 

 into an infinitely large lake. As the radius of the series of con- 

 centric ripples increases the wave-height decreases, till at infinite 

 radius the wave height will be infinitely small. The wave energy 

 has been so spread that it may be disregarded. 



The second law is usually worded, " The entropy of an isolated 

 system tends to increase." Entropy is a function which, while 

 theoretically of great value as indicating the direction in which 

 chemical or other processes take place, cannot be directly measured. 

 Further, one never has, in Biology, to deal with an isolated 

 system. The difficulty as well as the great interest of our science 

 depends on the close interrelation and co-ordination of all the 

 systems in it. A simple expression of the law, and one suited 

 to our purpose would be, " Every change takes place at the cost 

 of a certain amount of available energy." The amount of energy 

 " degraded " during a transformation from one form of energy 

 to another may be taken as an inverse index of the efficiency of 

 the transforming mechanism. 



States of Energy. 



A substance may be endowed with kinetic energy, or with 

 potential energy, or with both. Kinetic energy is directly avail- 

 able for work, potential energy requires the use of some kinetic 

 energy to liberate it. The energy of a substance may be in the 

 motion of the substance itself or in the motion of the ultimate 

 particles composing it. This kinetic energy and its value depend 

 on the mass of the substance and the rate at which it moves or 

 at which its particles vibrate. 



Potential energy, on the other hand, is said to be possible to 

 a substance in virtue of its configuration, i.e. position, composi- 

 tion, history, etc. A quantity of energy that may be measured 

 is stored up (or rendered passive in some way), and this same 

 quantity is theoretically recoverable in a measurable form. It 

 may not be apparent how energy is stored up, but it may be demon- 

 strated that it is stored and is recoverable. The simplest example 

 is the application of force to a perfectly elastic system. On 

 removal of the force the system will return of itself to its original 

 configuration, and an amount of work will be done by it, in 

 returning, exactly equivalent to the amount of the force of 



